This is not possible. Every message can have only one color pair (text color, background).
indicator has precedence before index normal color. Probably even before any other color (deleted, tagged, …).
You can set arrow_cursor
to see the color of a new message and also see where the indicator is. It changes indicator from whole line to arrow ->
at the beggining of the line. Indicator color applies only to the arrow, the rest of the line is colored according to your other color rules.
set arrow_cursor
To toggle the settings use macro, or just :toggle arrow_cursor
macro index,browser,alias,generic ,<space> \
"<enter-command>toggle arrow_cursor<enter>" \
"Toggle cursor arrow/line"