Have you started painting at all? What is your skill level?
If you have not painted it at all, I would suggest painting using a white undercoat and then using washes to get the colours you want.
So to get something like the above:
1) Undercoat white
2) Base coat the bits you want to be boney in a thinned paint; something like kaki/bleached bone
3) Turn the model upside down and hit the back of the model (the parts you want the darkest in a half strength black wash - try and also follow the contours of the muscles/scales so you extend the dark further where there are recesses. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
4) Allow to dry upside down
5) Repeat step 3 but extend the weakend black wash slightly further out - about half way between where you want the model to be dark and where you want it to be light - try and also follow the contours of the muscles/scales. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
6) Allow to dry upside down
7) Wash the whole model with a weak (50%) sepia/mud wash - use a wet, clean brush to gently clean the most raised areas on the "underside" where you want the lightest colour to be. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
8) Allow to dry upside down
9) Wash the darkest areas and the mid-tone areas in a weak sepia/mud again - try and also follow the contours of the muscles/scales at the edges. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
10) Now hit the darkest areas with a 50% green wash. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
11) Dry upside down
12) Hit the whole back of the model in a 50% green wash, use a clean, wet brush to "clean" and blend the edges as well as ensure the raised parts of the model are kept lighter. Use a clean, dry brush or tissue to wick off excess wash (if if it is pooling and dripping too much)
13) Dry upside down
Then you can hit the edges of scales etc in an appropriate colour, or completely repaint them or if you are careful you can avoid most of them when washing the skin. From my own experience of using washes you might be able to simply hit them with a different coloured wash to differentiate them slightly from the skin.
You can use similar techniques to paint the rest of the model, or paint them more normally.