Our render looks like below:
Time for some postprocessing. For educational purposes we will do the bigger part in ZBrush ;] Before we begin, adjust the colour values in Photoshop, it's much easier than with ZBrush's curves. Check the resolution and try to remember, then save image as PSD file.
Open ZBrush again, "Resize" the canvas, then "Document -> Import" Photoshop-corrected render. If canvas' and file's resolution aren't the same, the last one will be scaled (good algorithm, but blurry effect). In "Render" panel, select "Flat" type.
Take the tablet pen (with mouse this part will be MUCH harder), select "Highlighter Brush" tool, turn off "ZAdd" (and do this each time we will select any tool), set RGB Intensity to , Draw Size to  or lower and highlight the eyes where it's necessary - they should look 'more three-dimensional'
Then choose "Simple Brush", set Draw Size to very small, eg. . Use harder alpha then default ("Focal Shift" to soften too hard alpha). Set RGB Intensity to , select very dark green shade, and paint basis of iris.
Next, Draw Size to , RGB Intensity to -, stroke "Freehand" (lowest possible spacing), and green shade about center of palette. Draw the lighter parts of iris, and pupil. Highlight the lower part of iris, then add a reflection with RGB Intensity  hard "Simple Brush".
Colorize a bit left and right side of eye with "Colorize Brush", choose a tint close to orange. With "Simple Brush"/"Drag Rect" and proper alphamap add some details.