But you have to check the composition. There are some colors sold under the
same name but with different pigment used. Watch out for the student grade
either (e.g. W&N's Cotman, Grumbacher's Academy), although for reputable
manufacturer the substitute is clearly labeled, e.g. Cadmium Yellow hue,
Ultramarine hue, etc. (note the "hue", it means that not the real pigment is
used. Cheaper and fugitive materials are used. The purpose is for students to
learn painting only.)
However, if you carefully check the composition, you will find that both W&N
and Grumbacher have some colors with the same composition in the student
grade and the professional grade. However, you need to watch out for the
quality control factor. Maybe the student grade is a little more diluted,
maybe the pigment is not so finely ground, etc....
A good book for by Wilcox, the title is something like "The Best Watercolor
Pigment."
And for those who want to prepare your own color, a good book is "Color from
Earth." I forgot the author's name, but if you need the name, let me know
because I have the book at home. It is mainly for preparation of earth color
pigments for painters (watercolor, gouache, pastel, etc.), but it has very
good chapters on color preparation, grinding, etc.