I'm not an expert painter and so I don't know if it'll help but my own experience is also that a bit of mixing is required.
My process is as follows:
Base: Games Workshop Bestial Brown
Wash: any flat black that will water down
Tidy up extraneous wash: Games Workshop Bestial Brown
Mid coat: Vallejo Model Color Beige Red 804
Highlight: Vallejo Model Color Medium Flesh Tone 860
For buckskin I tend to use US Field Drab 873, then a black wash, then more 873 to tidy up and finishing with 873 + some off white. The effect is as per the leggings worn to left and right.