Do a 2 pastel pour side by side. One with only pastels, one with a dark color to make it pop.
If you want to see the details, you need to turn on the light
There are 2 reasons
Pretend you are stepping into your home after dark, and leave the lights off. You can still probably see the general outlines of your furniture, but you don’t see many details. As soon as you switch on the lights, you can see lots of details, like the mail on the counter. When you have a light source you can see more details in paint pouring too.
The light can come from an outside source, like the light fixture in your room or the sun, or it can also come from within the painting by using white or lighter colors.
The white paint also acts as a buffer between the colors. When you pour 2 colors on top of each other, they start to mix. The more things are “agitated” the more it mixes. Adding a layer of white, keeps the colors from going as dark and muddy.