As far as I've been able to tell, "truecolor" terminal support is about simultaneous display of a range of colors beyond what fits in a palette. Which, eh...seems a bit much for a terminal.
I vividly remember when I wrote DOS programs which used VGA/SVGA graphics in the early 90s; switching from palette-based colors to RGB colors was something I really enjoyed! Back in the 90s, loading a 256 color image that used its own palette into DeluxePaint would change the colors of DP's interface too.
Something similar would happen today under tmux, I think: if two side-by-side panels contain two different programs, and one of them reprograms the palette, the other program would be affected too.