Colored Items
This requires ItemsAdder 4.0.13 and Minecraft 1.21.4+ (both client and server).
To create a colored item you have to use a base texture, for example something like the quartz texture: minecraft:item/quartz.
You can use any texture you want, in this case I use the quartz texture as it's white and already included in the game files.

Then specify the color attribute in the graphics properties. In this case I use RED.
It accepts the following format:
RED(BukkitColorenum)#FFFFFhex (#is optional)13083194255, 255, 255(RGB)

Chaging color during gameplay
You can change colors using the command /iacolor <color>.
Templates and variants
You can create variants of the base item to make various variants of the same item but with different color.
Mark the base item as template: true then reference it using variant_of: xxx in the other items.
Example
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