I wish I had to knowledge to put a rainbow of color on this post but I am only good at making paper.
The simple way to obtain color is to start with colored shredded paper DAH! Other ways I color is with fabric dyes and I have a mulitude of plants that produce lovely colors also just putting torn bits of tissue in the white pulp makes a lovely selection , when doing this I don't mix the bits in I float them in the vat. If you want to decorate in other ways such as using glitter or flower petals they too are floated in the vat.
When I use dyes I do the PH balance and test after adding color I am not sure but I believe it coulld be adding minerals such as salt , better safe than sorry , wouldn't want my card to crumble in 99 years