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Now you'll make shadows. Add new, clipping layer. Pick soft round, dark-grey brush and draw some shadows on her belly and arms.
Change blending mode of created layer to Multiply. Decrease opacity to 50%.
There was shadows, so now you should take care of lights. Add new layer, also make it a clipping mask. Pick really light color. Use soft brush and draw lights on her hands, arms and head. Follow the light source.
For better blending, change blending mode to Screen. It looks more naturally.
Colors are still not good. try to fix that! :) Add adjustment, clipping layer with Gradient fill (set similar gradient fill as in step 19). Set it as is shown, then change blending mode to "Color" and opacity to 50%.
Again, add new clipping layer. Set brush, like from step 22. Use #257775 color, or - if you using other colors, pick shadow of color, which you used in step 22. Draw lights on model - arms, hands, breast, back.
Then, change blending mode to Screen.
Wow! That was long :). Now you should start working on mermaid tail. Add new layer. With hard round brush (I recommend pressure size), draw base for your tail. Keep calm - it's only shape.
Load "Silky scales" brushes (If you don't know, how to load brushes, go to step 12). Add new layer with clipping mask to tail (RMB > Create clipping mask). Pick dark blue color and let scales flow! :).
Click on created layer with RMB. Go to "Blending options". Then set "Bevel and Emboss" - Scales will be convex! :).
Add new layer. Click on it with RMB and choose "Create clipping mask". On this layer, You will create shadows of tail. Pick big soft brush and gently draw shadows. For making shadows, I prefer brush with pressure opacity.
Now add lights - new, clipping layer and light color. Remember about light source!
This step is totally optional. Repeat step 29. Draw bigger lights on tail. Then, just decrease opacity of this layer to 50%.
Back to layer with scales. Duplicate it (drag and drop layer to "New layer" symbol) and put on top of layers. If layers are unhooked, make it a clipping mask again.