Paint basic colors on the sketch, I use the gray background as the neutral color. After choosing each color, save it on a top layer
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Duplicate the sketch layer, one is merged to the color layer, one is hided away. Then start paint different shades of colors on each color region. The light is from the upper left corner. For painting, use the low opacity, flow, opacity jitter and flow jitter brush. Paint over the sketch strokes, don't delete them
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Start making the background layer
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Because I finish that in hurry, I didn't save one more step before the last one. I made the rain by making one more black layer, add noise, then use the moving blur, set by the direction of the rain. Then blend this layer to the lower layers by Screen mode. Duplicate this layer to make the drops brighter. Use eraser to clear the rain region below the "rain coat". Make the rain splash on top of the rain coat by scatter brush.