In order to have confidence to lay in some of the first washes it is always best to come up with a design plan. Spending time on that little rough sketch can eliminate hours of head scratching and problem solving down the road. Like a road map to your destination, it drives you through the journey to the center of interest or focal point.
So far in this study the “whites” and lighter passages are being preserved ahead of time. Masking fluid can often be used to save the little areas where you might place highlights or pure color later on, however if you can paint around areas rather than use masking fluid this is a better discipline. It isn’t always necessary to mask out the pure white of the paper.
If you’re aiming for interesting design contrasts, using opaque whites for details beside translucent glazes and color can have impact.