I know green traits mean they're carried, but why are some visible traits blue and some purple?

on genetically tested pets, traits that have a Hidden trait's genetics under the visible trait (eg, BC, DH, AB) use purple text, while traits that only have the genetics for one trait (eg, AA, CC, MM) use blue
Basically, it's to mark carried traits that are under a more dominant visible trait rather than under nothing.

for example, a sylvorpa (hijacking from guide)
Vis with Hidden
G1: OR [vis henna, hidden/carried clownfish]
Vis, no hidden
G2: EE [vis clown triggerfish]
Carry
M1: NI [carried sea fur]