They are formed by The triangles edges with creating matter. The colors are different because: It forms different Elements. They are: Absolutium (white) (9999.5th element) and Cloudium, which makes large clouds of Cloudium fog (blue) (-11th element). The reason there is so much of these elements is because of the density of Rainbow Matter
The Trippy Core
The Trippy Core is covered in clouds as a solid. It removes some of it (making squares) but it also transforms into Cloudium. The many bars shown as the picture are many The Absolute Triangles coated in Cloudium. The reason the core of The Absolute Square is so multi-colored is because elements from The Absolute Triangle mix into it due to density, as well as Rainbow Matter traces.
The main reason it is blue is because of the high density of the very blue Cloudium mentioned before. This material tends to radiate away into water and other materials depending of the isotope, but this is very slow. The natural fogginess of the clouds makes them great at hiding any Absolute Triangles you might have otherwise seen.
Rainbow matter is the most common thing in the Rainbowverse It is about 8 times more common than visible matter.