[i]Khaki would include a light rust color if the two shirts he is alleged to have worn were in sunlight.[/i]
Wrong!!..... Khaki is a YELLOW color.... Rust....Is a shade of RED.
Modern "khaki" has many shades but is basically a muddled color that is not specifically a primary color.
"a dull brownish-yellow color."
It was originally a term for uniforms ( "light brown fabric used primarily for military uniforms"), now referred to as "olive drab". Even if Brennan had that in mind when he referenced "khaki", there is the possibility that he could be describing a light rust color. Rust can be light reddish-brown (rust: "a reddish- or yellowish-brown flaky coating of iron oxide"). Brown itself often has red in it.