Assuming that the color just gets passed through to Tk for display, you would just write:
$n0 color "#FF0000"
(It's only in quotes because of the syntax highlighting here on Stack Overflow; Tcl only interprets #
as meaning a comment at the start of a command.)
The order of color components is red (2 hex digits normally), green (2 hex), blue (2 hex). If you're wanting to generate the value from three numbers in the range 0–255, use this sort of road-tested recipe:
$n0 color [format "#%02x%02x%02x" $r $g $b]