Changing default mail text color in MacOS Mail— I have a workaround.

Maybe someday Apple will fix this, but it hasn't happened yet. My workaround is simple. I create a document in Pages, type the text I want in a SIGNATURE (Hi, return several lines, Linda) select text and choose a color. (I like Grape.) Copy the text and paste it into a signature for my email. (There is a greater variety of supplied and created colors using Pages.)

When you compose a new email, the signature is there and in color. The only hitch is an empty line above "Hi," which I can close by placing the cursor in front of "Hi" and then the delete button or just leave it as is. If I don't start with Hi, then I just select the word and start typing. Hope this helps. This is what my new message emails look like: