Bumping this up because Snowth honestly makes a point.
Obviously in season 1, we saw quite a number of purple Anything Muppets. Then in season 2, the lavender muppet was introduced and they were still used, but not as frequently.
I think it was towards the late 70's to early 80's the purple muppet was being phased out, and it was such a surprise finding another purple muppet that DIDN'T have a Lavender muppet alongside.
I feel like nowadays the only time a purple muppet is used is when a character is assumed to be African American if they aren't using the brown puppet. It's clear the lavender muppet was its replacement (since it's basically a shade of purple anyway), and the purple muppet is now literally the only puppet from season 1 that doesn't have a live hand variant.