I will quote myself from another thread:
I made the following adjustments:
1) The Grey #78 with Beige #103 will never get you to the right color for the concrete. I used Testors Model Masters Flat Concrete, which is as close to the original color as you are going to get (IMO).
2) IMO Grey #79 is way too dark and will blend into the back panel. In fact after using this color and laying on the blue/green layer, I wiped the whole helmet and started over again. I used Grey #106 everywhere that it is recommended to use Grey #79. YMMV. In fact, I used Grey #79 on my armor and regret not using Grey #106. It just contrasts better with the olive greens. Many people would probably disagree.
3) I used a custom blend for the main green on the cheeks and dome. It was a blend of Green #78, Grey #106, and White #34. I can't tell you the proportions. I did it by feel. Start with #78 and slowly blend in the other colors until it looks right. The color is definitely lighter than Green #78. How much lighter you want it will depend on how your armor and gaunts look.
4) I used a custom blend for the main red on the mandibles as well as the purple / maroon. At the end of the day, Red #79 is really close and won't need much tooling around. I can't even remember what I used for the purple / maroon, but that is because I blended so many paints up trying to get it right.
5) I used POG (Panzer Olivgrun) for the left ear piece. I didn't spend a lot of time matching it to the original, so I don't really know how close it was. I just needed something that was somewhat close to the cheeks and domes but was off enough to be different. POG fit the ticket.
6) I could never find Humbrol 154 and replaced with Testors Insignia Yellow. Seemed to do the trick.
7) I used Valspar Metallic Silver as the base metal layer. Others (SuperJedi, for instance) lay on the metallic topically and have great results. Metallic is really fussy, dulls easily with repeated applications of masking agent, and dries very smooth making it hard for subsequent layers to grip well, basically defeating the purpose of the primer layer. However, I laid down the silver layer first for realism. I had chipping issues later on. I believe I read somewhere that in the actual helmet, the metallic layer went down first.
8) Weathering the helmet is a beast. Many guys did a far better job than I did. I did the black weathering with Humbrol 33. There are leather colored splotches all over the helmet that use, I believe, something like Humbrol 62. I used it mainly for weathering the stalk, but check out pics of helmets done by Fett4Real and Major. They are real beauts in this department. I ultimately opted out as a bad weathering job over an otherwise good looking helmet paint job is a risky proposition.
As a tip, buy some glass mixing bottles from the hobby shop to keep your custom mixes. It makes the blending process work more easily and if you ever have any damage or need a touch up, you can go back to the original blend.
Finally, I am in no way an expert. I did this once and liked the results. But other guys have done this a bunch and are professionals, so I will defer to them (guys like SuperJedi, F4R, Major, etc. etc.). The above color combos seemed to work for me.
If you're in the United States and you're buying them from Humbrol.com, the answer is yes. I believe you can also sign up to be notified when they're back in stock. I got all of my paints through that website.