Now that I have been teaching for about 5 years, I would say the No. 1 topic painters struggle to get a handle on to move forward with their painting is contrast. For many it feels unnatural to create such dark shadows and such bright highlights because it isn't how we see surfaces and lighting around us on a daily basis. What is often glossed over in teaching and discussing contrast, is that with miniature painting we have to create an optical illusion with our painting.
I know it sounds weird but hear me out. Miniatures are scaled down representations of creatures or objects we have some real world analog for. Yes, I know, there are no real mermaids but we have fish in real life and humans in real life. Therefore, a mermaid has real world analogs that we can use as reference for painting. Same goes for all fantasy creatures. In someway, everything resembles something we know or can identify with.
Since these representations are scaled down, our lighting and colors also need to scale in order for our brains to be able to read the piece correctly. Try looking at an unprimed model from a distance and you will see that without paint or any sort of contrast on the model, it becomes difficult to discern exactly what you are looking at. This is the case for models that aren't painted with the appropriate level of contrast. They come off looking "flat".
In this instance, flat means that there isn't a lot of difference in value between shades, tones and tints. This is where contrast comes in. We need to make sure that not only our highlights and shadows contrast on each surface, but that when picking our color scheme, the base colors of the surface of the model contrast with each other. That means we can't paint an entire piece in just middle values. We need to have light colors, dark colors and middle value colors.
So, how do we check that we have contrast between the surfaces of our model before shading and highlighting? By using a camera trick!
If you have a smart phone you can take a photo of your work. Go into the Edit mode on your phone and find your Saturation Setting. Click it and completely desaturate your photo. By doing this we remove the hue and only observe the value (relative lightness or darkness).
In the photos above, we can see how this trick is used. The first photo shows a side by side of the model with just thin basecoats on and the airbrush primer job. The second photo is the first photo completely desaturated on my Samsung Phone. The third photo is the final paint job and the last photo is a desaturated version of the final paint job.
In the second photo we can see that all of my basecoats are about the same value. Once the color is removed from the photo, Danerys looks completely grey. There is no difference in the value of the colors used in the basecoat. You can also see that just looking at the thumbnail of the image it is really hard to see details of the sculpt. This allowed me to see how I needed to proceed with the paint job to create more contrast between surfaces and within each surface.
With the final paint job and increasing the contrast, we an finally see all of the details of the sculpt and accentuate certain aspects of it to make it stand out.
There is a reason why I have specified how to desaturate your photo. You don't want to use a Filter on your phone. Nearly every phone has a Grayscale Filter but it alters the photo as opposed to only removing the color so you can accurately see the value of the color. Make sure you are following the correct procedure when trying this trick.
Here are some more images of models with a good level of contrast to them.