From what I understand initially ranks were on shoulder epaulettes (if that's spelled right), and would have the epaulette be one color and the pin a single color. As uniforms evolved the ranks of LT and Lt Col both got an 'upgrade' and it simply became tradition. That's a very brutish explanation I admit.
Once when I was yelling at my underling in the army, he got so pissed off he tried to throw me with a minesweeper. Luckily he didn't come up with anything more expensive! xD It was a pretty tense week!