One of the meanings of the term study is "practice piece" (it can also be used for an experiment to test composition, or a quick painting to capture the essence of a scene for later work). The reasoning behind doing a study, rather than a full or 'real' painting, is that you focus on one particular aspect of a subject, and work at it until you get it 'right'. Then when you do start the bigger painting, you (in theory) know what you're doing. This saves the frustration of struggling with a small part when you want to be working on the whole painting, and means you never end up with one section of the painting overworked (which can can look incongruous).
The small sea study shown above was painting on location, or plein air. Although I had an array of colors available (see list), I used only Prussian blue, cerulean blue, cobalt teal, and titanium white.
Prussian blue is a favorite of mine and is a very dark blue when used straight from the tube, but quite transparent when used thinly. The section behind the wave, and the lower half of the wave, were painted with Prussian and cerulean blue. The top section of the wave was painted using cobalt teal, and the wave foam with the titanium white. The darker blues show through the lighter wave colors because I was using the paint thinly (glazing) in places, blending in others, and applying it quite thick where I wanted solid color.
The aim of this study was to get the angle of the wave and the change in color on the wave right, as well as to create a feeling of moving water. Having got that working to my satisfaction, I could then focus on painting a wider seascape.
