I would paint in straight color until you're comfortable enough to accomplish the same result in a different way. It's going to be harder to understand color, value and tone if you're also having to deal with blending modes and multiple layers.
Value studies would help you understand how forms appear under different lighting conditions. Tonal studies would help you understand value/hue matching. Of course, color studies are going to help you understand color.
"Astronomy offers an aesthetic indulgence not duplicated in any other field. This is not an academic or hypothetical attraction and should require no apologies, for the beauty to be found in the skies has been universally appreciated for unrecorded centuries."