The two paintings by Monet & Van Gogh have different moods, for the most part, by the use of color.
The basics of colors: they can be saturated or unsaturated, light or dark, and warm or cool. This is all you need to know about what defines the colors that you use. But, there is much more about the use of the colors.
All these things that define colors are relative.
One color can be more saturated than another, but it may still be a gray and unsaturated, just having more saturation than another.
The light may not be a value #10 and the dark may not be a value #1. The range between light and dark could possibly be very subtle, maybe as little as 3 different values.
Warm and cool is relative also. All the colors in a painting could be reds, but it is relative.
Some reds being warmer than other reds, some cooler.
Color has the strongest emotional affect on the viewer of any of the elements in a painting.
The use of color can be used as: