Fibres have to run through a pretty harsh process during dying, higher temperatures, motion, water.... Is ist possible to mix dyed and and undyed colours of the same yarn or will they behave noticeably different in the finished piece?
This is probably more an issue with wool than with silk?
Is it safer to pop the undyed skein in hot water first?

I know the safe answer would be to make a swatch. But I'm quiet sure here is someone with experience😊

Commercially dyed fibres will be fine with white of the same fibre/manufacturer. I wouldn’t put any yarn in hot water, I prefer to soak anything handwoven (other than tea towels ;-) in warm water, with gentle soap.

Sorry, I should have mentioned I'm referring to handdyed fibres. With acid dyes for protein fibres should be reached a temperature of at least 190/ 85 Grad for around 30 minutes.

In the last newsletter I saw you sell undyed silk too at JST.
Silk doesn't shirnk like wool but what is about wool or wool blends?

I would trust that the exact same commercial yarn will be behave the same, whether it is dyed or labeled as undyed, however they were dyed... unless noted otherwise. If one color did not have the same opportunity for shrinkage after wetting or fulling as another, there may be differences in the remaining shrinkage after using them together for weaving. Personally, I wouldn't have any concerns unless it was yarn that someone else had spun, or yarn that was noted on the labeling as the exact same but produced many years apart. If some yarn was dyed and not rinsed, or some was still in the grease or undyed, I would skein or reskein, wash and dry the yarn before doing anything else. (I would do the same for really old yarns of uncertain history, because I prefer to see any weaknesses beforehand. It's not fun to watch yarn disintegrate into pieces while warping or weaving.)

Well I have woven a blanket in 2 different colors in the same line of product and it fulled a little differently. one color consistently performed differently than the other color. It was slight but it was there.

Thank you all so much for your answers!
I just asked in the mill I ordered my yarns. They wrote back the yarns should behave in the same way as the undyed yarn was washed and steamed prior to use anyway.