Heterochromia definitely exists sometimes on Roshar. It seems to occur occasionally when a lighteyed and darkeyed person have a child.
In Words of Radiance we meet Redin. (source with spoilers:)
He is the illegitimate son of a light eyed man and a darkeyed woman. He does not seem to be considered noble and gets confused reactions from people due to his eyes.
Another example is one of Graves' children. (This page has general spoilers for Stormlight and possible minor spoilers for other Cosmere works:)
Graves' child was the only one with his wife who was "one-eyed" even though they were a darkeyes and a lighteyes in a relationship. It's likely not a common occurrence, but we do see a few characters with this.
It's not likely that anyone would see the child as noble, because it is evidence that one of the parents is darkeyed and thus non-noble. It may also be cultural depending on where in Roshar you are at.