Adjustment and Fill Layers
Two useful features in more recent versions of Photoshop are adjustment layers and fill layers, which make it possible to add a variety of controllable effects to underlying layers. Effects such as Brightness/Contrast, Hue/Saturation, Channel Mixing and Color Balance can be placed onto these separate layers, as well as solid colors, gradients and patterns. A key advantage is having the ability to overlay effects onto your piece without having to bake them onto the image, so you can keep changing effects until you are happy.
Imagine you want to try out some different color effects on this image without ...