Last update of Mutt (to version Mutt 1.5.23 (2014-03-12, Gentoo 1.5.23-r6)) introduced a problem with my current color theme. For Mutt prompts, such as "Open mailbox:" or "Purge deleted messages?", the prompt text wasn't visible. The foreground and background colors had both changed to the same dark color, and only highlighting the prompt would make the text visible.
The change to color handling wasn't documented anywhere that I could see so I went straight to the source. There is now a new color object called "prompt", so the prompt can be made properly visible by adjusting to color theme like this:
color prompt white black
which would make the prompt text white on black background.