Even if they're not consistently color-coded, the negative side will generally be the SAME color. So, whether they're all black, or all white, or all brown, etc., assume the wires, in the pairs, that are all the same color, will be the negative side.
On the other hand, it wouldn't be that uncommon to find front panel LED connectors that are themselves wired wrong (with reversed polarity). So, if you find one that's connected properly, but not working, just flip it around.
Also, if I'm referring to a pin diagram in a manual, I always want to compare first, to confirm that the markings right on the motherboard --the labels next to the pins --correspond exactly to the diagram shown in the manual. (You can really waste a lot of time, if you're referring to a manual that shows the wrong diagram.)