Why is Hoid’s color aura from his Breaths not noticed by anyone on Roshar? Are the characters just not used to noticing that sort of thing, or does he not have one?
There are a mixture of reasons. Not the least is Hoid's control over investiture.
Would someone with breath be able to notice his aura on Roshar? E.g. Vasher?
Various methods of detecting investiture would still work, depending on how good a job Hoid is doing of suppressing his.