I understand your powdercoater's concern about mixing powders.
I here the same thing from the various manufacturer reps. The
problem with what they are saying is when you order a custom
color from Tiger Drylac or Dupont etc., they gladly mix THEIR
powders together to come up with the custom color you want
then charge you many hundreds of dollars for the effort.
I can tell you from personal experience you won't have any problems
with color consistency as long as the two powders are compatible
and are thoroughly mixed.
I looked at the Tahitian Red powder coat color match as a challenge.
To arrive at the final color for my project, I coated dozens of sample
panels with different powder combos and ratios checking for ompatability
(color consistency). I then had the samples that passed, tested with a
spectral analyzer and compared to a NOS "Tahitian Red" side panel to find
a match. This is the same thing the manufacturers would do when you
order a custom powder, that's why it cost an arm and leg.
To see the results, check out my thread "79 CR250 progress pictures".
Look at the last pic, the closeup of the Tahitian Red painted tank and
see if you notice a color difference between the tank and steering
head (where the light is the same on both).
None of this is all that important at this point because the RAL 3020
you are using is super close to Tahitian Red and your frame will look
"like a million bucks" when you get it back, and that's all that really
matters.
Post some pics when you get a chance so we can see how it turns out.
dogger