Some terminals like xterm allow to redefine the colors via resource files or dynamically, and if it's exposed correctly in the terminfo entry for $TERM, you can do it with:
tput initc 4 1000 0 0
Change ANSI color 4 (normally blue) to RGB (1000, 0 0), that is bright red.
If the terminal doesn't support redefining colours (see infocmp -1 | grep initc), you can also, for applications that use terminfo, trick them into sending different escape sequences to request colour 4 (blue).
infocmp -x > terminal.info
Edit terminal.info, replace the name of the terminal with something like "myterm", and edit the "setab" property (set ANSI background).
Instead of
setab=\E[4%p1%dm
That is:
tput setab 4
sends ^[[44m, change it to:
setab=\E[4%?%p1%{4}%=%t1%e%p1%d%;m
The %? ...., is an if-then-else, to say send "1" when asked for "4" and the requested one otherwise.
So
tput setab 4
will send ^[[41m, (red) and tput setab 5 will send ^[[45m
Then, compile it with:
TERMINFO=$HOME/.terminfo tic -x terminal.info
and use it as:
TERMINFO=$HOME/.terminfo TERM=myterm the-application-to-trick
That only works if the application uses the terminfo database to send sequences to the terminal.
Best Answer
I would just use the Color Picker tool from Gimp, which will let you click anywhere on the screen and will give you the RGB value for the color at that point.