Substrates(what you are dyeing) is seldom a perfect white. Dye colors are transparent so you can see the color of the substrate thru the dye.
Tien's has estimated that her substrate- 30/2 silk has a Munsell Notation of 2.5 PB 9.5/0.5. (For information about the Munsell notation see Wikipedia.)Then she dyed this silk with Lanaset Sun Yellow dye as several different DOS. Do you see any effect of the off white color of the silk in the color of the dyed silk? Would you expect to see a similar effect on say violet?
Translating the Munsell numbers:
2.5 PB 9.5/0.5: this means that the base color is a purple verging on blue (that's what the PB is about), value 9.5 (nearly white), chroma 0.5 (not very different from neutral).
What is interesting about the Sun Yellow is that it initially starts out very high chroma (in the Munsell notation, 6.5Y 9/16 means yellow hue, light color, very intense color) but as you go down to 0.5% and 0.1%, the chroma drops radically from 12 to 4 to 2. In other words, the intensity of the yellow drops off hugely in pale colors.
This is roughly what you'd expect if you are overdyeing pale violet-blue with yellow. Violet-blue and yellow are nearly opposite on the color wheel, so adding a little bit of violet to a little bit of yellow should produce a nearly-neutral color, i.e. not very bright. And you can see that as the concentration of yellow decreases, the intensity of the yellow drops off as the bluish undyed yarn color starts to show through.
The advantage of the Munsell numbering system is that it gives you the terms to describe a specific color exactly, and a calibrated set of chips to match colors to. This is handy because when two colors are very close to each other, sometimes it's difficult to say, "Oh yes, that's obviously yellower than the other", but matching them up against the chips to get the Munsell numbers makes the difference more obvious. There have been a couple of times when I've been scratching my head saying "There's something wrong here but I can't quite figure it out." Then I compare it against the Munsell color chips and say, "Aha! This one matches 5 GY 9/10 and the other one matches 5GY 9/12. So the chroma is too high and I need to add some black to dull it down a little."
Overall I've learned a huge amount from this study group, and highly recommend it if Karren does a second one!
I agree whole heartedly with the weak yellow combined with weak bluish purple, what we get is a yellowish grey. It is striking to see the chroma jump from 4 to 12! This is an unusual color for a substrate, usually they are more yellow. The effect is also most pronounced with the sun yellow because it is weak tintorially, gold or mustard on the same substrate, also dyed by Tien:
Here the jump in chroma occurs at a lower DOS, meaning that is takes less gold to overwhelm the purple in the substrate than yellow. This shows the tintorial strength of each dye. But once overwhelmed the chroma does not climb very high meaning that gold is a dull color compared to the much brighter or saturated sun yellow.