When crafting weapons and armor, your item must match the color Aster you're crafting with.
You may only use one color of Aster to craft a piece of armor or weapon. You do not have to have matching colors on each armor piece, though! For example, you may make a Blue Aster Helmet and a Black Aster Chesplate for the same character. You may not, however, use both Blue Aster and Black Aster to craft one helmet.
Non-Aster aspects of the Armor (fabric, accessories, magical/electrical components) may be any color. Just keep in mind that the Aster itself with the proper colors needs to cover approximately 50% of the assigned armor region.
You may color pick any shade off of the matching slider to color your armor, unless you are crafting with Rainbow Aster. Rainbow Aster Armor can be any color, it does not have to be picked off of a slider.