How do I make color materials into a single image file? I created multiple cubic objects with solid color for each of them to group into one big cube (To import into Roblox Studio). I want the texture to look like this...

How would I do that? When I upload it into Roblox Studio the object is completely gray, but the object shape is there and also size. I searched on the web and couldn't find anything to solve my problem. Please Help!
What you need is BAKING. I recommend looking up some tutorials on texture baking in blender. Ideally, you should have used some color pallet and put uv faces on it to assign different color to different faces and using only a single material. you can then do a cleaner unwrap and bake the textures to this new uv map to get the final texture.