While I have a personal preference for black and red, I do understand how that causes problems on certain monochrome devices, so the light and dark browns in the default are fine, or FFDF00 and C19A6B (the piece colors in Hundred Acre Chess) if that doesn't hurt the eyes too badly.
Edit: It turns out 660088 is too dark. Try 8000FF instead.
While I have a personal preference for black and red, I do understand how that causes problems on certain monochrome devices, so the light and dark browns in the default are fine, or FFDF00 and C19A6B (the piece colors in Hundred Acre Chess) if that doesn't hurt the eyes too badly.
Edit: It turns out 660088 is too dark. Try 8000FF instead.