Hurrah! I've found the answer. My scrotwm open up xterm with tmux instead of plain xterm. Just now I used:
program[term] = xterm -e tmux
To open new term with tmux. That's what causing the 256 color issue. Seems like opening tmux like that made the xterm not setting the correct $TERM. That's why if I open an xterm, then open tmux, the 256 color will look as it's intended. So I changed that to xterm -tn xterm-256color -e tmux so that the xterm will open with 256 color $TERM correctly. And it works! Here's my ~/.scrotwm.conf for your pleasure. Feel free to use it.
Do note that I don't use spawn_term = xterm anymore. Initially I changed that to open tmux automatically, but defining other than "xterm" value just don't work. Hitting MOD+Return just didn't open any xterm at all. So that's why I had to use program[xterm] instead. As long as it works.
And here's my whole ~/.profile. Do take note on the upper part of the file especially on the xterm & tmux bit.
As usual, use it if you want to. Later.