Claude Code UI colors appear paler/desaturated inside tmux vs raw terminal

Description

The Claude Code welcome screen (mascot, UI elements) renders with visibly paler/more washed-out colors inside a tmux session compared to a raw iTerm2 pane. The mascot's salmon-pink color and other UI colors appear less saturated in tmux.

Environment

• OS: macOS 15 (Darwin 25.4.0, Apple Silicon)
• Terminal: iTerm2 3.6.9
• Tmux: 3.6a
• Shell: zsh
• Claude Code: v2.1.83

Investigation

I investigated whether this was a true color (24-bit) passthrough problem in tmux. It is not.

Terminal color capabilities (outside tmux):

• TERM=xterm-256color
• COLORTERM=truecolor
• TERM_PROGRAM=iTerm.app

Tmux client terminal features:

256,bpaste,ccolour,clipboard,hyperlinks,cstyle,extkeys,focus,margins,mouse,osc7,RGB,sixel,strikethrough,sync,title,usstyle

RGB is present — tmux is correctly passing 24-bit color through.

Color gradient test (run in both tmux and raw terminal):

python3 -c "
rows = [('R',1,0,0),('G',0,1,0),('B',0,0,1),('W',1,1,1)]
for name,r,g,b in rows:
    print(f'{name} ', end='')
    for i in range(77):
        v = int(255*i/76)
        print(f'\x1b[48;2;{v*r};{v*g};{v*b}m \x1b[0m', end='')
    print()
"

The gradients appear identical in both environments — smooth with no banding. True color rendering is confirmed working in tmux.

Conclusion

The color difference is specific to Claude Code's own color rendering. Claude Code likely detects the terminal environment differently inside tmux (e.g. checking TERM, TERM_PROGRAM, or some other heuristic) and selects a less vibrant color palette as a result, even though the terminal is fully capable of 24-bit color.

Inside tmux:

• TERM=tmux-256color (set by tmux's default-terminal)
• COLORTERM=truecolor (forwarded from outer shell)
• TERM_PROGRAM=iTerm.app (forwarded)

Expected behavior

Claude Code UI colors should look identical regardless of whether the user is inside tmux or a raw terminal pane, given that the underlying terminal supports true color either way.