Mostly by combining different hue and saturation to the original image - if you have Photoshop, it's fairly easy to do. Alternatively, you can create new layers and use the brush tool to manually apply an appropriate colour with just a touch of transparency. As for the eyes, it's a matter of using a different type of brush setting to create the intensity, then adding a layer style that imbues them with just the right amount of glow.
That said, it's easier to do in Sheena's case because she doesn't use the same sort of eyes as were seen with Thrawn, in which the entire eye was a glowing red, and not merely the irises.