I've managed to work a few things out about the Yokomo springs.
First of all, the spring chart from Petit RC is a great resource and gets a big
from me -
http://www.petitrc.com/reglages/BuggySpringChart.pdf
Secondly, Yokomo's coloured BX springs are very close in rate - they have only 0.25 coil difference between them. This may be a good step for a short touring spring but on a buggy it is far too small to make any difference. A 1/4 coil is worth 2-3% on the rate at these lengths, the AE and Losi springs are at closer to 10% difference between each rate. So as suggested above you need to go right down to a blue or pink to notice any real difference.
Thirdly, the front black springs are quite a lot stiffer than the nearest rate, at 0.75 coils stiffer than the next coloured spring. And are still at best a "medium" spring, similar to an AE silver.