1. Make a new layer and hide the other layers.
2. Design the tattoo on this layer using full opacity black. Make sure it's not too detailed, because the small details won't be apparent in-game (unless it's on the portrait).
3. Unhide the other layers and change the blending mode of the tattoo layer to Overlay.
4. Move the tattoo around a bit to get a nice position. Consider how it will mirror if it's on a part of the skin that is mirrored (back and front of arm/both arms back and front/ only front/etc).
5. Reduce opacity of the tattoo layer if there are parts that appear pitch black.
6. If the tattoo becomes very saturated (for example, on a brown surface, the tattoo tend to look dark red), duplicate the tattoo layer and set blending mode to Color. Reduce opacity until you have a subtle, realistic colour.
7. Touch the tattoo up, in a new layer, with freehand. Add some soft, thin highlights around it to enhance the contrast between the tattoo and the skin.
Deleted member 157129
Ugh, frog-perspective screenshot! xD
Anyway, not much has changed so far so I can't tell how it's going to look, but I can tell I like the style you're going for. Much more refined and clean compared to your older works (at least so far).
Looked around a bit for a tutorial on adding tattoos to a texture, but i couldn't find anything decent. So I'll explain how I do it briefly (in the next VM).
Low opacity, dark red works. If you don't get the result you want, you can alternatively use tertiary colours around the main colour, so if you're using green as main, instead of using red, you can use lime green (tertiary colour between yellow and green) or aqua green (tertiary colour between blue and green).
Deleted member 157129
Response to chat whisper.
Black and white as shading is unnatural. Black can be used in certain situations to emphasis an outline, but in most cases, steer clear of black and white. Use complementary (tertiary and secondary) colours to shade with. If main colour is blue, use orange (secondary) in shadows. If main colour is red, use green (secondary) as shadow. I think you get the picture. Basically, opposites on the colour wheel complement each other.
Use primary colours for important parts of the skin. Use complementary colours to shade with. Use tertiary colours for less significant elements of the skin, and shade with complementary, tertiary colours. This of course not "the correct" or "the only" way to proceed with art, but I think it's a good basis.
mhm.. the hair should be following priest's model more, and it shouldn't end up cut like that along the face.. try using some hair brush to draw out a few strands everywhere hair goes near the face. also do the same along that hair edge thingie, so it doesn't look like just pasted there..
chest.. well i'm unsure what you want to achieve there..