Prime the walls with white primer, then start testing colors. I think the glossy foam boards you have are not the right kind, try another store.

You're correct about the current wall color getting in the way, unless it's white.

You can use poster board from the dollar store. But if you need to paint/prime the wall white anyway, you might as well just paint very large swatches on the wall.

The BEST way for most ....https://samplize.com/

Cheap foam boards from the dollar store painted with primer and then 2 coats of the test paint. I like using the boards since you can move them around and to different rooms in different light

The problem with painting directly on the wall is that when you apply the final coats, that patch of paint can telegraph through--not just the color, but also the texture. If you've painted patches on the wall, be sure to sand thoroughly, feathering into the regular wall color, then prime before painting.

That's why foam boards are better (and their movability).

The best way? Paint one entire wall, and use a trimmed area. Yes............the whole wall. It's paint, not a hole in the wall. Better yet is to paint TWO walls.

Very few people excepting pro's get get any great idea from circles, squares, foam boards etc. A foam board is very useful when painted 2/3 the paint color, and one third your trim.....and yes, then you see the true color. You can put that in a tote, drag it around when looking at tile, fabric etc. But in a room.....you take your best guess and have at it in a large enough, clean enough space.

You are 100% correct in that everyone "sees" color by comparison. Therefore the only comparison you want to see is your trim paint, if any. Roll it hon, just roll.It's paint.

Another vote for samplize.com

Get poster board. I either use the ones already cut down to a smaller size or use the big ones & cut them down myself. Paint two coats. I too have seen a lot of people on here recommend Samplize.