HLD is not pure pixel art.
It starts to become a little difficult for some of us to really comment what is appropriate if the whole thing is going to be changed with overlays and lighting effects.
But providing some examples of with and without ur intended lighting effects would be a good place to start.
As it stands i find the colours really boring and the designs lack 3d form.
For example the walls are all just flat with a few little bumps and lines and dont have any real structure to them.
There's some little projection issues too like the number of pixels the door recedes at the bottom doesnt match the top.
I'm only viewing on my phone's 5" display @ 2,560 x 1,440 and at 100% its still noticeable.
I'd encourage u to literally think outside the boxes and "paint" in some larger structural parts.
Do some design work, use references, watch Aliens and Dark Matter and stuff to get ideas.
By the way the wall above top ramp, the light fall-off is rendered very boringly.
Try to vary the size of colour bands in these situations, same goes for rendering cylinders and spheres.
Someone posted this recently which is a good overview of what I'm talking about with shading -
https://youtu.be/V3WmrWUEIJoAnother thing that helps is have more actual forms where there are lights instead of flat things . Scifi is perfect for this because u can make all sorts of strong lights and darks and different colours to an extent u can only really get away with in this genre so make good use of it!