Mine are changing color as well. Read my post, just a couple down from yours about Heuchera Color.
I have 4 different varieties and 3 of the 4 are turning an ugly brownish w/green underneath...it's a hard to describe color. I should try to post some photos.

The green heuchera need less light. As for heucheras with color dark leaves need sunlight to keep a deep color. Most of mine including the blacks are in direct sunlight. They just need more water. You should move it or put it in a pot to be moved around for sun.

Also, I believe new growth may be truer to color then it matures to green.

If I remember correct Palace Purple always matures to green, it is only the fresh new growth that is purple.

I shovel pruned it long ago when better purples came along.

This is mine, it is always this color except when the heat really hits. Then it turns the ugly browning unless I water, water, water...sometimes I do some years I don't.
I have had it since 2007 doesn't seem to lose it's color no matter where it is planted.
:)Aqua