The easiest method for doing this is to start with a color picture. Draw a mask around each eye and go to Object > Create >Object: Copy Selection. This will make a duplicate of the eye. Be sure to make duplicates of each eye (assuming you did one eye at a time). Select the original full image of the face, and go to Adjust > Image Adjustment Lab. Take the saturation slider all the way to the left - this will take all the color out of the face and leave you with a grayscale image. If you want to adjust the eyes, re-select them, and go back into the Image Adjustment Lab. You can play with the saturation, contaratst and brightness sliders for the fastest way to acheive the results you are looking for.
If you are begining with a black and white photo, the process would be similar, in that you will need to mask the eyes and create a duplicate object of the eyes. However, on major difference any variation in color in the eyes, or any black areas (i.e. pupils) or highlights (white reflections) will have to be handled manually and drawn in seperately. Once the eye objects are masked just like you need them, then you could fill the object, or paint on the object with the desired color. If this becomes fake looking, and looks like two bright blue blobs, you could use the transparency slider on the Object Docker. You could also try changin the Merge Mode on the slider to get the effect that works best for you.
This last method will also work if you are starting with a color picture with someone with brown eyes and want to chnage them to blue. Hope this helps.
