Vince
I've had a go at scanning the available references and colour chips for the 1943-44 variant of RAL 7028 Dunkelgelb. The result isn't definitive, that would require a professional scan of one of the original RAL reference cards. Nevertheless, I've scanned the relevant Chory chip, which is currently the most reliable source available and compared the result with scans from the Real Colour book. The result seems to be pretty consistent.
The picture shows a couple of usable bits of information.
Firstly, the nearest modern RAL colour is RAL 6013, Reed Green. How close a match? At the bottom is the Delta E value of 4.98. CIE 2000 Delta E is a measure of colour difference and can be interpreted as:
On a typical scale, the Delta E value will range from 0 to 100.
<= 1.0 Not perceptible by human eyes.
1 - 2 Perceptible through close observation.
2 - 10 Perceptible at a glance.
11 - 49 Colors are more similar than opposite
100 Colors are exact opposite
Of course, the perceive the difference on your model, you'd have to hold a chip against it. You might think the colour is close enough.
If you want to refine it, you can see that Reed Green has more red, green and blue than the original RAL Dunkelgelb, which means it's a slightly different hue and more saturated ie intense. Your colour mixer might be able to tweak the values using these figures.
At the top is the Munsell colour description for the scan. I couldn't find a translator for Munsell to NCL conversion but your chap might be able to use Munsell. I can also give you a RGB Hex value, which is another measure.
Is the colour too dark? I don't think so. On a large ie 1/6th model, it will appear lighter. You can see this if you get a sample of RAL 6013 on your computer and enlarge it to fill the screen. It seems lighter in bigger swathes. It's not what we're used to seeing but it is a close representation of the original, as documented by Chory, who has so far been the only person to offer to the world painted chips rather than printed ones.
You could always get a sample mixed to test and after applying it, you could play around with filters to adjust it.
All the best.
Stephen