Here's the official reply from the Wildflower Center:

We got a similar question very recently from someone else about the color
variations of the Indian paintbrush (Castilleja indivisa). It is a matter
of genetics. Probably your plants didn't mutate; but, instead, carried the
right form of the color genes that combined with similar forms of the genes
from another plant to produce the new colors. You can read the answer to
the question we wrote from before at:

http://www.wildflower2.org/NPIN/FAQs/Detail.asp?ID=326


Thank you very much for your question and your photographs. We hope you
will contact the Wildflower Center again with your native plant questions.

Very best wishes,

[snip]