It is called chromatic aberrationThere's quite a few ways to achieve this effect, so you can google it for some tutorials, but one of the easier methods is to take your finished painting to Photoshop and go to Channels menu(usually found near your Layers, depending on your set-up). It'll show you 4 channels, RGB, Red, Green and Blue. Select one channel (either Green, Red or Blue) and reset the RGB channel to visible (eye icon) and carefully drag the layer you selected with the move tool a few milimetres in any direction. Say if for example you selected Green, the greens of your images will be slightly shifted, causing some of the edges to show green. (When you're done, don't forget to select the channel RGB again, otherwise anything else you do/add will only affect the greens hahaYou can a lot more than just moving it and you can use history brush or various other methods to make sure it only affects the areas that you want, but this is just a quick and easy exampleI suggest looking it up on google for even better tips!