Adding dark world palette to specific light world areas?

Is this possible? Since the base for my hack has a mosaic transition on every screen I figured it may be possible to apply the dark world main colors like for grass to a certain area. But could I also apply the opposite to several maps in the actual dark world so that they have the light world tileset?

Yes, it is possible. Try to copy paste a part of the dark world into the light world and adjust the sprite gfx and the main blockset number and pal. Now only the hardcoded part will remain, such as main grass, and the "frame" of objects (light green by default= same as grass).
 
All the remaining palettes must be changed via the palette editor in HM,however this part is not entirely documented. But you should find the colour for grass (where it is green by default and not brown) and hills.

I believe you might've misinterpreted what I was asking. I want to apply the hardcoded grass color of the dark world to an area in the light world and apply the hardcoded grass color of the light world to dark world maps.

This is also possible, however all the green grass in light world can be edited, not just one area of it. The death mountain and east palace areas are exceptions, and can also be reverted to green.

You can also change the dark world grass colour. It is all in HM under palette editing.

I remmember, I once edited the grass in GoT to a little more dark green, but then had problems with the frame around objects, which remained light green, which can also be fixed similar as in PW (you can see the frame in HM but not in-game).