The cause of many different shades is using transparency, and the design anti-pattern of using a strong colour and toning it down with transparency.

Starting with the national parks, since I've just added them to brighmed:

One of the better styles I've seen for national parks is a strong outer line and line inside the border, and very light green shading inside.

The US NPS uses this

As does OpenStreetMap Carto, which uses it only for national parks and nature reserves, not leisure parks.

For webmap cartographic conventions, it varies.

• Google has switched around a few times, using all possible combinations of showing trees and showing national parks. They also change for the different style variations and zooms.
• Michelin does a light green fill for parks and shows no physical landcover features
• Stamen Terrain shows terrain and no national parks

My preference is for the NPS style if we're going to show national parks. It makes the quasi-administrative nature of the polygon clearer and the boundaries, and also works much better with parks that have water inside them.

This was to be done in the new style stuff, which is no more, so it's now an issue again.