Look at these covers (text specifically):
Do you see it?
"Mario" word. First, letters go as green (M), blue (A), yellow (R), red (I) and green (O) in SMW and YI.
Starting from Mario 64, they start going blue, green, yellow, red, green. It's Mario 64 and two Galaxy games (Sunshine skipped it entirely and had all of it's title written in ocean blue).
Starting from Land colours start changing again. Now it's red, green, yellow, blue and green again.
And finally we arrive at 3D World which ditches second green in "O" for pink.
It's kinda interesting that letters kinda match playable characters: red M for Mario, green A for Luigi, yellow R is for cat Mario (?), blue I is for Toad and pink O is for Peach.
I am doubtful about yellow I, but pink O is obviously there because of Peach. Pink was never there previously so it's not just a random colour rehashing on their part.
Probably doesn't mean much, just a random observation. For all we know they could just decided on letter colouring on cover to cover basis and designer just picked the ones that are the most pleasing and colour balanced with rest of the picture.