Don't sweat the rolled-up trousers.. That was pretty common as it was cooler than blousing the trousers into the boots..
As for the dark color, don't sweat that either.. They got dark when it was that hot, drenched with sweat.. They other thing to consider is to stain the trousers, at least from the mid-thigh down, with a reddish-brown color.. Khe Sanh was located in the Central Highlands, and that was pretty much all red clay..
Note the trousers on the following Marines. Some are bloused, others aren't, others are at "half-mast", and pretty much all of them are lacking any "black" on the leather portions of the jungle boots...
You can tell this 155mm crew hasn't been here long..
Note that chip-straps didn't dangle, but were instead snapped around the back of the helmet..