I DO use lighting all the time and rarely use PBR. What you are trying to do SHOULD be easy. So rest assured you do not need to switch to fancier materials or anything.
A black ambient is also totally fine.
The weirdest part to me is that it works in the scene explorer but not in the application.
As long as you have a white directional light and a properly setup material then things should work. I guess unless you have some code somewhere that is corrupting the ColorRGBA.White “constant”.
You could try explicitly giving your DirectionalLight a color “just in case” it either requires that for some reason or your ColorRGBA.White has somehow turned black.
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BTW, you can post code directly in the forum instead of taking screen shots of it: