Well all colors of Rock are Plymouth Rocks and meet the same standards. At two weeks you begin to see which have fast growing tails and fuller wing feather. But serious culling for sex is harder in the yellow legged varieties. I wouldn't even begin to make up my mind into six or eight weeks in that variety.
And in Columbians culling just for sex is a mistake since color quality (of hackle and tail) and body type for the breed is KEY. You pair down early at your own peril.
Some of my slower growing pullets and roos turned out the most impressive in type and size.
You shouldn't begin to cull for quality until laying at a minimum. Be soooo easy to throw out your best birds sorting any earlier. That said if you're near E. TN, I'd be interested in a few.
You can early cull for way too many comb points, for multiple crooked toes, but that's about it.
Columbians won't even put in full hackle and tail color til very near POL.