Like anything else in Photoshop there are probably multiple ways of doing it.

The most simple way I can think of would be to select the part of they eye you want to change - I would use the lasso tool, but the oval marquee may work as well. Zoom in close on the eye and use a bit of feathering on the selection, depending upon the size of the image.

Use Image->Adjustment->Hue/Saturation [Ctrl+U] and tug the Hue slider to whatever colour you want. From this tool you can also increase or decrease the saturation for fine tuning. Remember the setting, select the other eye and do the same.

If you have a problem remembering the settings, or are going to be doing a series of pictures the same way, you can use the [Save} and [Load] buttons to save and then load the settings for each eye in the series.

You might try the "magic wand" selection tool (select one pupil, shift+select the other pupil). Then go to Select>Feather and set it to about 3 pixels. You can play with that for best results.

Larry is right about using the Hue/Saturation tool. Anything else is too heavy-handed for this....