First of all, I really like the style of your map. Very vibrant coloring. What I know about shading with water pencils is that you first give the surface you want to shade, a base color. This should be the lightest color in that area.
You then take a darker colored pencil and fill in the part you need to shade. If needed repeat with even darker colors.
When applying the water with a brush, always stroke the brush from the light area towards the dark area or else the darker pigments bleed on the lighter area.

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