The utility map is a specific map used mostly for masks. Here is a step by step explanation.
How to use the hemoglobin mask (R):
1. Start by selecting the Red channel of the utility map, copying it.
2. Create a "Fill" layer, and make it a red hue.
3. Paste the copied red channel into the "Fill" mask.
4. You obtain an image that masks only the hemoglobin of the face.
5. Using levels on the mask we can narrow down the details and achieve a much more convincing mask.
6. Set the mode to darken.
7. Finally change the opacity and achieve a result like this one.
How to use the melanin mask (G):
This mask is mostly used for changing skin tones, but you can get creative and use this mask for other purposes.
1. Start by copying the Green channel of the utility map.
2. Create a hue/saturation layer.
3. Paste the copied melanin map into the mask of the Hue/saturation layer.
4. Contrast the mask by using levels.
Here are some examples using the melanin map: