Sankaku Channel/

^Those are tag types:

Tags can be assigned a type. Currently these are the possible types: artist, character, copyright, general, medium, meta and studio. Types are assigned by editing the tag directly or by applying a type through tagging, which is done by submitting the tag with a prefixed type, for example “character:edogawa_conan”.

There's a description for each tag type on help: tags.

Pink is studio and aqua is medium.

To check a tag's type you can go to tags and search for the tag, the tag type is the last column in the resulting table.

I don't know if this is explained anywhere, but I'm pretty sure I'm correct.