When we think of contrasts in painting what comes to mind is dark and light value contrast, or cool and warm color temperature contrast, or the contrast of hard and soft edges.
A real important contrast is thick and thin paint. The thickness of the paint, when we apply it to the canvas, goes a long way to suggest form and depth as well as enhance sunlight and shadow contrast.
When we mix a color for a sunlit area the value and the color temperature is really impacted when we use thicker paint. I can mix a color for a light area and if it's not thick enough it won't have the impact of sunlight, even if I mix the right temperature and value. Thicker paint stands out more and looks lighter than thin paint.
As a learning tool, thick paint forces you to simplify (it's hard to get too detailed with a brush full of paint), and helps to understand color better. It's easier to understand the impact of a color with thicker paint.
Just like watercolor is designed for transparent washes, oil paint is designed for thick opaque brush strokes.
The painting below is a detail from a painting from the Teton Mountains. All the paint is thick but the light areas are thicker so they stand out from the darks. The thicker paint also makes the brushstrokes more important. I'm thinking about mass and value more than detail or trying to render objects.
This is a detail from a John Carlson painting. The darks seem to recede from the thicker lights. The stronger colors have more impact because of their thickness.
I painted this in southern Utah and used the thicker paint to show more form. The brushstrokes follow the shape of the trees and hills to accentuate their shape or direction. This also helps to keep things simple. When using photographs to paint from, it's more effective to think in terms of the light brushstrokes following the form, it keeps me from being too literal with photos.
At first it can be hard to paint thick enough. Our tendency is to paint thin if we're unsure, so use a palette knife or painting knife to mix your paint. It's hard to mix thin paint with a palette knife.