The proper answer is not as far as I am aware. I bought a NOS '80's Conquest with yellow springs in original packaging and the leaflet didn't give any indication that the spring rates were different. They did seem slightly stiffer than the modern Champion Flyer I bought for the Moonie, but the leather was a fair bit thicker on the NOS Conquest as well so the stiffness could be down to that.
I’m asking as I saw a yellow springed Conquest on a bike a charity bike shop is selling and was lusting after but I have the physique of a racing snake. I don’t think they’ll split it but worth asking....
AFAIK all the springs are the same, just the colour that's different.
I had a red version and changed to black...no discernable difference, just not so cool.
Gratuitous 1985 Highpath shot:
And the 1985 Overbury's Cross Fell (Yes, it really is that slack!):