My highlighter defaults to this ghastly greenish yellow color. How can I change the default to a different color?
This applies to macOS. Don’t know if it is the same in Windows.
Click and hold the colour swatch to open the colour palette. Select the colour you want and Scrivener should remember that colour until you choose to change it again.
I know what you mean! It’s possible to change the color by right-clicking on the highlight button, and that selection will remain during the current session of the current project. But in a different project, or in the same project after closing and reopening Scrivener, the color reverts to the icky default.
In the Windows program, under Tools > Options > Appearance there is a dialog for selecting colors in various areas of the program. But I looked pretty hard and couldn’t find one for the highlighter! That’s surprising, but it seems to be the situation. If someone else can point it out, I’ll be glad too!
Absolutely right. It may only be an aesthetic issue but this is a writing program for aestheticists. That dunny-green pen icon is worse than a Microsoft paperclip, It’s constantly in view reminding me I’m going to splash pea soup over my lovely clean words if I go anywhere near it. I don’t even use the highlighter now that I’ve got to grips with the notes function.
Please Scrivener masters, can you fix it for our highlighter choices to remain constant between sessions? 
Being able to keep your choice of standard colors up in the top bar would be a boon.