I know that the Darlings were in at least 1 color episode, as was Ernest T. Bass. But were they in any more color ones?

What about Thelma Lou? Was she in any of the color ones?

Ernest T. was only in one color episode, as were the Darlings.

Thelma Lou appeared in one color episode, "The Return of Barney Fife" from season six.

Thelma Lou appeared in one color episode, "The Return of Barney Fife" from season six.
Ah yes. Was that the one where they go to the school reunion or something, and Barney is hoping to get things started again with Thelma Lou, but she shows up with her husband?

Yep.