when drawing and painting, landscapes and portraits, to start with the dark colors first or the light colors first, also when it has to slide over each other
There is no correct way - all approaches are valid. Some will be more appropriate for a particular medium e.g. light to dark is usual for watercolour.
I tend to work from middle values outwards, but it can be helpful to establish core shadows early to provided a framework to build on.
It’s common to paint in a ground colour first, and I would consider what the overall key is (how light or dark the image is as a whole) when choosing the value for that.
1000 thanks for the help
It just occurred to me I said core shadow when I meant occluded shadow!