in the eu.pb4.polymer.api.resourcepack.PolymerRPUtils#requestArmor(Identifier) function, colors from (1, 1, 1) to (255, 255, 255) are used to register my custom armor with a vanilla dyed leather armor. but why only odd rgb number are used?

wiki said that there are 73.4% sRGB being used in the dyed leather armor. Could we use the rest 26.6% to register custom model? or use them first

Because you can get any color with commands. It works just fine and doesn't cause issues from my testing anyway, but it would be trival to change that