What should I do if one of my babies eyes is yellow?

Same as for 2. Many babies will have a transient yellowing in the white of their eyes the first week after birth as their body learns to metabolize bilirubin. If this is what you are seeing & it is worsening, the baby needs to be evaluated. If your baby is older & the yellow is new, the baby needs to be seen to see what the problem is.
Be Seen. If one of your infant's eyes is yellow, it is unlikely to be due to a systemic cause like jaundice. Some parents will describe their child's eye as yellow when crusted tears or pus collects in the corner of the eye. The crusting of tears is not generally a problem, but the possibility of infection makes this a condition for which evaluation by a medical professional is recommended.