Wet vs Dry
Often when artists discuss watercolor painting, you might hear terms like: "wet on wet", "wet on dry", "dry on dry" and so on. These terms sound more complex than they are.
Generally, since watercolor is a water-soluble paint, you can use several techniques simultaneously in one painting, and people can't always precisely determine which technique they're using. Sometimes the paper is dry, and sometimes it's wet from previous washes.
The key difference lies in the paper preparation. And the paint, as we already understand, is always wet, hence the names of painting methods.
There are two ways to prepare paper for painting:
Dry method: attach dry paper to a board with adhesive tape/
Wet method: soak the paper in water until it becomes like a wet sheet and then place it on the board without attaching it.
Regarding the first layer (see article Three Layers), when painting on dry paper, you'll need to work much faster because the paper dries quickly.
When painting on wet paper, the paper will help you extend the process for a much longer time, but it will also take much longer to dry.
The advantage of painting on wet paper is that even if you apply paint and leave the sheet to dry, the paint continues to blend in the paper's pores until it's dry, and as a result, the colors of the first layer end up richer.
After the first layer dries, both methods follow the same process for additional layers.
Summary
"Wet on wet" or "wet on dry" depends on the condition of the paper before you start painting.