Reddit reviews Making Color Sing, 25th Anniversary Edition: Practical Lessons in Color and Design

You shouldn't have any problem mixing brands - most are similarly formulated! Some brands like M Graham or Sennelier even have honey in them and can be used with other brands without problems.

This website explains how watercolors are made! If you look on the back of your paint tube (the WN cotmans will have this) you should see a few letters and numbers ie PB 29 - this is the pigment that is in tube (in this case ultramarine blue) and you will find that different companies W&N, M Graham, Daniel Smith, etc all have their own ultramarine blue but all of them are derived from the same pigment - PB 29. "Pure pigment" tubes will list just one pigment, but often companies will make colors that have multiple pigments - ie Payne's Grey is PBk9 and PB29.

Handprint.com is a great resource but a lot of reading, wetcanvas.com has a great watercolor forum and handbook with a wealth of information. A book that really helps me mix and understand color theory better is Making Color Sing by Jeanne Dobie.