You're probably barking up the wrong tree a bit asking me on this topic, but I can give you pointers here n there.
HD is the man you want to get tips from, he is the master!
If you want, I think Hertz wrote a little back a few pages when I asked him, he seems pretty open to questioning...
I would feel bad trying to push my limited knowledge like as if it were gospel, I don't think I'm that great at it. I draw a LOT of inspiration from HD.
But thanks for appreciating my efforts.
Basically; what Hertz said to me was to pick one, two or three 'main' sponsors and pick some colours that will make up your base design.
Keeping in mind where to place you main sponsor decals,
Start with a basic base design which you can edit as you go on. A lot of good designs just use a simple base. Though HDs are in another realm when it comes to that.
Then it's just a matter of having some creativity in placing smaller sponsor decals.
Hertz will tell you to ALWAYS use yellow