Change blending mode to Overlay, decrease opacity to 20%.
I'll show you, how to make your image deeper. Add new layer. In your color picker set green and black colors. Then, from top menu choose Filter > Render > Clouds.
Change blending mode to Soft Light, also decrease opacity to 20%.
Then, from top menu choose Filter > Blur > Gaussian blur...
This step is optional. Add new layer. Pick Sparkles brush and draw some sparkles around your mermaid. Use white color.
Set blending mode of this layer to Soft light. Layer will inherit colors of the rest of image.
Again add new layer. Use the same brush (Sparkles, white color) and set bigger size. Draw some strokes around tail.
Now set blending mode to Vivid light. Decrease opacity to 33%..
You should fix Brightness/Contrast. Add Adjustment layer.
Optional - add Hue/Saturation adjustment layer. It will fix all colors.
In your color picker set as primary color some green or blue (as your wish!), add adjustment layer with radial Gradient fill.
At the end, change blending mode to Multiply.
Thank you for following this tutorial! You are amazing. Write if you have questions! :). Personally I am graphic designer from Poland. See my portfolio here: msriotte.tk. I'm also on Facebook: here!- msRiotte