Here's another tip for adding noise, which will make the skin look more realistic and match the rest of the person's arm.
1. Once you've painted the skin tones the way Hawkeye suggests, create a new layer above it.
2. Over the same area that you painted the new skin, paint on this new layer with 50% Gray. (You can create a selection with the Lasso tool and then click Edit>Fill>50% Gray. The selection doesn't have to be perfect.)
3. On this new layer with 50% Gray, change the layer blend mode to Overlay. The gray disappears and it looks like you've accomplished nothing, but in fact you've created a base of pixels on its own layer that you can now add noise to.
4. Go to Filter>Noise>Add Noise. The amount of noise (percentage) depends on the size of the photo and the grainy-ness of the texture you're trying to match. Your photo is fairly large, so try somewhere between 5% and 10%, and be sure to check the box that says monochromatic. At first, the noise should look like it's somewhat too much... as if the person was at the beach and leaned in the sand with a wet arm.
5. Now, give the noise a slight Gaussian blur. You can experiment with how much, but perhaps 1 pixel will do, just to break up the sharp edges. It's a bit of trial and error.
6. Once you've blurred the noise, now reduce the opacity of the noise layer. Start by reducing it by 50% and then see if you think that's too much or not enough, and adjust it further from there. Keep playing with the amount of blur and opacity until the resulting skin texture closely matches the rest of the person's arm.