Here is an example of a digital illustration done in Photoshop. I took an image of Angelina Jolie (courtesy of the World Wide Web) and modified it to turn her into a mermaid. I enlarged her eyes, changed her nose, added fish scale skin (via layering an enlarged image of a salmon), colorized her in many layers using the brush tool, and blended in a few different textures. She is absolutely gorgeous! The octopus tentacles were selections from another image, modified with puppet warp.
This image below was my first rendition. For the final one on top, I added more octopus hair and removed the seashell hairpiece in the upper right corner. I also reworked the fishscale texture a bit and made it curve along her jawline.
The green skin tone is accomplished by a teal/green paint layer added with the brush tool. If you turn that layer off, you see the colors that are accomplished mostly via textures.
And for reference only, here is the original image of her. I am not sure who took this image and cannot give credit to the artist!