I've done some of my rifles in "HK"-inspired Duracoat colors. They seem to vary quite a bit, from the pre-production sample that was widely seen at trade shows, to the actual production runs in the wild (more of an OD-green hue). A good reference is "The Reptile House". So if you're going for that kind of HK tanodized look, realize that it's much darker than DDC, and actually looks OD green out in the wild. And the lighting of available pictures leads me to believe that it looks very different in natural light as opposed to indoors. So honestly it's hard to pin down exactly what it looks like. But then I'd not worry about the color(s) being an exact match either. And it is varying shades, even on the same rifle. The uppers always seem darker than the lowers. So whoever you have do it, direct them to your reference photos and let them know what your preference would be in terms of the variations. Mine tend to run closer to the OD colors seen in the wild.
For engraving, I use Scott @ THSF. He is very familiar with all the engraving that goes on in the industry.
For the color mixes, on the lowers, I use Duracoat "WWII Olive Drab" as a base, with "Magpul FDE", Magpul "MCT", and "RAL 8,000" mixed in to taste. On the uppers, I use either Magpul FDE or MCT as a base and add in the Olive Drab or a Woodland Green, with a smidge of RAL 8,000. It does not take much of the green or OD to create a nice OD-green color, which contrasts slightly with a more Olive brown looking color on the lower.
On the optics, furniture, n such I use straight Magpul FDE for a nice off-set. The rail I will shoot more towards the upper color, but still give it a little color variation to keep things interesting. Looks better than DDC IMHO. The colors all combine to give you a natural camo effect.
I know you wanted Cerakote, but these colors are fairly easy to replicate in any system, so this will give your guy a starting point. I like a flatter camo coating, but if you want the more anodized look, have him throw in a "Bronzine" metallic in the mix.