Weekend Wonderings: What kind of colourants would we use for shampoo or conditioner or liquid soap?

In this post, Can I use BTMS-25 in a solid conditioner?, Baby Kat asks: Very interesting point. So far, I have always used btms 50 and it works well for me. I wanted to ask, what kind of colorants would you recommend to add to shampoo/conditioner/liquid soap? I find it more appealing when the color of…