I super agree that the tutorial needs a lot of work as I also didn't realize this until a few days into playing and processing everything. I think a lot of the confusion also comes from the starter potions being neutral. I know there are user made guides for newbies, but I think a lot of people miss those even though there is now a Guides forum section. (pixpet.net/forum.php?section=topics&f=18&title=Game-Guides For future reference if you're looking for guidance on something, there might be a guide for it here!)

In the tutorial the section that talks about a Pixpet's colour doesn't mention potions either -

"Color: The color of the Pixpet. A Pixpet with the color "cyan" can only find other Pixpets with the potential for this color. Also it can only find cyan colored items. There are 16 different colors. Please note that the pixpet color does not have an influence on the actual appearance of the pixpet."
(Even the cooking section of the tutorial doesn't really state that Red Colour pets can only use Red potions, it just says that the ingredients effect the potion colour)

It sucks that you made potions you can't use, but you can sell them! I bet if you stuck them up for auction, people with pets of those colours would be interested in buying them. c:

There is a mega topic on site suggestions, so I'd probably advise posting any suggestions on tutorial improvement and the like there - pixpet.net/forum.php?section=posts&t=145&title=Site-Discussion---Ultimate-Idea-and-Improvement-Thread

EDIT: Forgot to ping hopefully this works! @Cat