I might be able to help a little here, as I have only ever used Games Workshops Citadel paint range. Citadel paints are a water based acrylic and do not require thinning or diluting before use. You can use them straight out of the pot and onto whatever your painting. They are pretty much touch dry within 15 - 20 minutes in a well ventilated room and fully cured within 24hrs. I've found the only exception to this rule is the metallics they do. They take a lot longer to dry and tend to cure completely within a day to. I believe the reason being is due to their composition. GW do a range of inks/ink washes to, but I'm not certain of their composition either I'm afraid.
I've never used any of Tamiya's products, but I believe they require thinner of some kind?.. could it be that your ink wash is not suitible for use on top of a water based acrylic paint?
GW
used to do a yellow ink.. but I dont think they produce it anymore. If you havent already done so JL.. I would suggest having a look at GW's website as it contains a full list of their entire paint range. It might be an idea to ask the staff at your local store for any expertise they might have. Many of them dont just collect/paint Warhammer items
If you get chance. Take the item your trying to ink wash yellow with you, that way they can see what affect the Tamiya ink is having on their own brands silver paint.
Hope this is of some help to you JL!!