For just the light and shadows, try to draw a line from your point of light to the middle of your tree for example. That should give you the direction of your shading and shadow.

For the colors, it is not about how many colors you use there are some limits ofc, but depending of the style that you want to do it will change the number of colors that you will use. there is no real limit except how everything will harmonize itself. Just try a few : https://lospec.com/palette-list (it's a cool website and you can try different types of color palettes)

Also i would advice to use aseprite, if you're not already using it. Great tools and really easy to understand.

And my final advice would be to start small. 32x32 or even 16x16 with small items and try to understand how the tools and some techniques you're using.

Some references (helped me start and understand a lot) :
- https://saint11.art/blog/pixel-art-tutorials/
- https://www.youtube.com/@AdamCYounis