This is the part where I tried a new (to me) approach to layer masks. Earlier I've either painted a completely new image in layers on top of the sketch, or created a mess of layers merging with the sketch to make the final pic.This time I copied the flattened sketch to several (4) new layers, and made layer masks for each copy. If you're fairly new to masks, in this case they mimic the matte painting technique used in movies and animation. It's a great way to separate objects in the foreground from the background.To help me see what I'm masking out, I use a 50% opaque layer with a bright blue fill between the layer I'm masking and the background.I mask all the layers from foreground to background. These layers will now act as a base clipping-layer (new PS CS term) or group (older term). In essence, anything painted on a layer which is on top of the masked layer, and grouped with it (Ctrl+G) will only show what is within the base masks borders.Masking can be done with either lasso or a hard brush. Using a soft brush is usually not a very good idea, if your mask has a very complex outer edge (for example the grass in this image), it's probably better to make a rough mask as a base, and add grass to an ungrouped layer on top. Alt-clicking the layer mask in the layer palette helps you see if you've missed any spots.Some of the internal details get their own masks to make sharper outlines.