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?
Sun Yellow
DOS 0.1%
10Y 9/2
DOS 0.5%
7.5Y 9/4
DOS 1.0%
7.5Y 9/12
DOS 2.5%
6.5Y 9/16