I got a few brown jugs to store chemicals. The jugs came with plastic caps with pressed foam liners as seals.
Wouls the foam interact badly with black and white chemicals, or am I overthinking this?
Probably overthinking it.
Speaking for myself, I've never had a chemical problem that I could attribute to the type of seal on the bottle. I have lots of different sorts of bottles with different sorts of seals, they all seem to work.
i use brown bottles as well - over the years i've lost all the foam seals (the paper ones are still in the top and brown stained). i've never replaced the foam ones and never had a problem. i do use a protectorant, ( tetanal, argon gas?) after filling.
Both the Tetenal kit and the Ilford chemicals I got came in bottles with cardboard seals. I don't know if our friends across the sea have heard of molded in conical seals or not. But anyway, in both cases the chemicals really degraded the seals, even with the foil left on with just two small holes poked in it. With the Ilford developer the seal was destroyed and the unmixed chemicals had to be transferred to a new bottle.