Use gradients and Hue/Saturation to apply a rainbow to your portraits
You need to first select the eye area to apply the effect. Using the Lasso tool (or any selection method you prefer), draw around the inside of the eye. Once the selection is made, click the New Layer icon from the Layers palette.
Pick the Gradient tool from the Toolbar and then click on the Gradient Editor area in the top Options bar. Pick the Spectrum gradient preset and then click OK. Drag across the selection to add the colour. Repeat until you get a pattern you like.
Things are too harsh at the moment, so scoot down to the Layers palette and click on the blend modes drop-down menu. Pick the Colour mode to merge the gradient with the photo.
Go to Image>Adjustments>Hue/Saturation and use the Saturation slider to tame things further; simply drag it to the left to reduce the effect. It might be that you want to go further in which case slide it to the right.
Staying with the Hue/Saturation, you can also use the Hue slider to alter the colours in the eye. Simply move the slider left or right to get the effect you want. This is such a simple edit but makes a massive difference to the final result.
Press Ctrl/Cmd+D to deselect the area you had selected. Zoom in and check that the colours haven’t seeped out anywhere. If it has, pick the Eraser tool from the Toolbar and wipe away. Once happy, repeat the process for the other eye.