This is one of those "it's up to you" calls, but my preference is to have built-ins match the trim color. If you opt not to do that, I think I would try to go noticeably different from the trim so that it looks intentional. The good thing is that since it's paint it's changeable if you hate it.

so the prefabs are pre cut in 9' (some are 6' or 8') lengths and kitchen counter depth...24" or so, I believe. they also have island sizes at 3x5 or 4x6' sizes, and the edges are simply finished. here's a close up of some,,, this is Python (click on the pick and you'll get to see the finished edge) they also have quartz (a little bit more. maybe 500 or so this was at a different stone yard and these are rounded edges. more quartz the fabricator brings them to the residence and cuts the holes for sinks or stove tops. some fabricators my come out and template your kitchen before hand if they have to cut for an L shape. Once at the home, they install them just like they would any other granite. just as an example, this was a kitchen we did for a rental about 7 years ago. the L shaped perimeter countertop and the island were all pre-fab granite. for the granite alone, I paid 700 or so? maybe less? i can't remember. and then had the guy come out and cut it to fit was another 1000 or so. so the granite for that entire kitchen came less than 1800 or so. it was very nice granite, nothing wrong w/it and the fabricator did a great job w/the cuts and the seaming. these are cheaper than slabs because they come pre-fabbed and are limited in types and colors. They still have plenty to choose from though. I live in SoCal so there are hundreds of stone yards and tile places around here. Depending on where you live, you may have this, or you may not.

I would not paint the peninsula a different color. The wood cabinets look nice, they add warmth to the space, are you sure you want to paint them? Have you considered getting a panel to cover the side of the fridge and pulling the fridge top cabinet forward so it looks more built in. I would prioritize that before painting the cabinets.

@Lucia Wilke I haven't gotten around to it yet, but I've decided to just do my island. I'm also going to replace my backsplash. It doesn't show in pictures, but it is an ugly white that doesn't match the rest of the kitchen.