I am not one for mixing and matching granites. Yes it can look really good, but it is not easy to pull it off.
I personally like the look of all the granite being the same with the cabinets being different on the island and perimeter.
In your case you have lots of dark elements so your designer is suggesting a way to lighten things up.
Can the island be changed to cream? Then you can still use the tropical brown or find a cream granite. Personally, I would not go too wild. Your tropical brown has a tight pattern, but it is a pattern not solid.

I tend to agree with ellendi. I don't like to mix and match granite, especially since the cabinets on the island will already be different. I think you can go with different cabinetry on an island, or different granite, but not both. It would just be too busy visually. I like her idea of a cream cabinetry island with the same granite you already have. Black and brown is sometimes difficult to pull off together.

I would try my darndest to not do brown on brown and brown on black. I think the Tropical Brown probably isn't the best choice here, and that's what is throwing things off with the black island. Something lighter for both would be a better choice. Look at something cream with a bit of brown and black in it.

Thank you so much for your input! I would *love* the look of a beautiful light-colored granite all around to tie it together and lighten it up, but we already have the tropic brown granite on the perimeter and don't plan on changing it at this time (we just installed it about 2 years ago - at the time we weren't thinking we'd do a more extensive remodel). Also, the island cabinets are already ordered as well, so those are black for sure.

I do really like the look of black cabinet islands and brown cabinets together, especially with black appliances. We've got good lighting in the room, and the floors will be natural oak, so those will be light as well. But I just feel like now we're stuck with the countertop choices for the island. Ugh, boy do I wish I had looked at more pictures and posted here before we placed the cabinet order for the island!!

Sooo...since we are not able to change our cabinet color choice at this time, would you just try to find a matching tropic brown slab? I found some images (links below) of brown and black cabinets with a similar tone granite as the tropic brown. Or are there any other options we should consider? Like is there a granite that is almost a solid cream with not much pattern so it doesn't compete with the tropic brown? Or something else?

https://www.houzz.com/photos/rustic-kitchen-rustic-kitchen-phvw-vp~82720-Kitchen-traditional-kitchen-other-metros

https://www.houzz.com/photos/giants-ridge-project-1-traditional-kitchen-phvw-vp~1494212-Ridge-Project--1-traditional-kitchen-other-metros

https://www.houzz.com/photos/kitchen-8035-traditional-kitchen-columbus-phvw-vp~557737-8035-traditional-kitchen-columbus

https://www.houzz.com/photos/custom-arch-cabinets-traditional-kitchen-minneapolis-phvw-vp~568367-Arch-Cabinets-traditional-kitchen-minneapolis

https://www.houzz.com/photos/kitchen-8035-traditional-kitchen-columbus-phvw-vp~557745-8035-traditional-kitchen-columbus

Oops - I meant to respond to the suggestion for cream cabinets for the island as well...
In general tend to prefer the look of the island cabinets being darker rather than lighter than the perimeter ones, but because of the other colors in the kitchen I can see how cream would have been a beautiful choice and would have tied it together! I suppose down the road I could always paint the island, but right now I know I couldn't bring myself to paint over brand new black cabinets.

I looked at all your examples and I can see why you like this combo. However, it looks like the granites are either the same or very similar. Most had dark counters.
Maybe the tropical brown could work on the island.

I am planning walnut perimeter cabinets and a painted island. I had originally planned to do a different counter top on the island but I am re-considering this. In my case, I am doing walnut cabinets and an olive tone island. I wanted a walnut butcherblock on the island but am reconsidering because I think it may be one finish too many. In my case we are 50% DIY so I am just waiting for the cabinets to be installed before I make the final call. I understand you probably don't have that flexibility.
The first Houzz link you posted has the wood perimeter and the black island both with the tropic brown. It looks really nice. Do you dislike it? Based on that photo, I feel that if the island had a lighter, granite with a lot of movement it would definitely be too much for me. However, that kitchen has a lot of ornamental detail on the cabinetry and is very traditional whereas my style is more clean lines and modern, so that kitchen is already very busy for me and I need the island to be calm.
What if you printed out that photo or another one that best approximates your cabinetry and a picture of a light granite and cut and pasted a mock up. Sometimes getting out the arts and crafts supplies is the best way for me to visualize. Crayons are very beneficial.

Have you considered a wooden countertop for your island, in the same tone as your granite? How will you be using it?

Perhaps try a simple honed black granite for the island?

Or my second choice would be to stick to the Tropic Brown to match the outer cabinets/countertop.

These two choices would create a cleaner and more cohesive look to the whole space. You could add lots of interest to the island with a trio of hanging pendant lights.

I would have done the island with more contrast to the brown. I have contrasting cabinets (white and knotty cherry) and contrasting counters (marble and black granite). It is not as easy as you might think to get a good combination. In your case, it could be expensive too (calm marble in creamy tones and similar granites can be very pricey). You might want to look at quartz, butcherblock, even stainless can be a fun and less expensive island top. If you have the time, you might look for a marble remnant or salvage piece, but the length may be difficult to find.

I'm sorry it's taken me so long to respond - it's been a crazy week with the remodel. I wanted to thank you all for your responses!

bronwynsmom, I love the look of butcherblock counters and that would be a perfect choice to go with the tropic brown, but we need something very low maintenance and durable. We aren't careful cooks and we've got toddlers.

lascatx, I'm staying away from marble because of staining, but I like your idea of a creamy granite with a similar look as marble. Does that exist? We'd be open to quartz too - I haven't really looked into that as much, but we should probably explore that option also.

teacats, honed black granite is something our KD suggested as well. I really like the look of honed, but am not yet sold on black counters with black cabinets. Do you think it would be too much black or would it work?

localeater, I do like the first link with the tropic brown, so I'm definitely still considering that!

Luckily, we do have the flexibility to wait and choose the island granite once the rest of the kitchen is done, so I think I'll have to do that before making a final decision. Once I'm at that point, I'll post some pictures for you all to see and maybe get some additional input then! Thanks again!

I wonder if you can find stone or porcelain tiles that are similar in color and texture to your cream tumbled stone backsplash? It could tie your island to the perimeter, without adding another surface to the kitchen. With a 2 x 6 ft island, that would be only three 24" sq tiles. Porcelain would be better because it would not stain.

Florida Tile has many choices of porcelain tile that mimics stone. They even have polished marble looking tile that is really porcelain. The lines that are meant for either wall or floor tile even have liner that you can use to edge the island. Or save $$$ like I did and get a composite wood edging and paint/stain it.

Here is a link that might be useful: Florida tile

I have tile that's called 'cracked stone', similar in color to the Florida tile. I don't know the brand, could be Dal tile. It's a porcelain tile rather than a ceramic.