Hello,
I’m trying to make a game using every roblox built-in material…
Except, I don’t know the best brickcolor choice for each material.
Could Someone help me out on this?
I would say for Concrete its the fossil colour, I use it sometimes for my games
but for fabric, I would use bright red, although the fabric texture is very dark for bright colours
but for others I think its random
Not sure, it mostly depends on what are you building and how are you building it.
I would just say that on fabric is better to use brighter colours, concrete looks great with wheat colour if you would like to build a house, for grass you can use dirt colour and you can have pretty cool dirt and you dont have to find some textures (a bit away from topic, but saves time).
Others pretty mutch like glass have best colour on icon, but most of them depends how you like it yourself, just try to play with colour id changer and you will at end of the day find best colour 
I sincerely think there is no set colour. I think you can make different materials with different colours. For example:
More people might go with a reddish colour for bricks, but with a light gray, it becomes the inside of a public toilet. (atleast for the ones we get in the UK)
There are variety of colors that you could go about this, whatever color you choose it should fit the theme and style your going for this question should be something that you should answer yourself research around and see what style your game features then implement those colors.
The general common colors, red, orange, yellow, and more are something that could be used for brick materials whatever type of style your game features, find different color schemes and see which ones works best for you. Having a sorting style is important so you could select the correct colors that’ll fit on that object or building don’t just throw colors that don’t match the theme.
If you intend for your game to have variety of colors go for it. At least in first choose the common color test and work around which color work on those materials however it should match the style your going for i suggest testing is a good option to see which colors work on each materials.