Sophisticated start. You could go with black and white photography, architectural pieces, art pieces- anything you love on the wall. Then, add pillows in the colors of the art work. If you want to stay more monochromatic, think in terms of textural interest, such as a shag area rug, pillows with seam detailing, etc.

Either will be nice, especially with a speckle through it, the black will be more dramatic, it depends what suits you the most, I myself would go for black as I like contrast and I'm a bit of a drama queen lol

I understand going with the grey since it is an 80's style house. You could also do bright bold colors. I do not know if that is your style. I have attached a quick rendering of the gray scheme you mentioned. I prefer light colors above dark colors. This keeps in theme with the sky and earth. Makes the house seem more grounded. The dark entry also draws your eye to it especially with the light coming from the other side of the house. I have also attached a quick rendering of a blue scheme. It looks like you are on a beach and I thought this would blend in with the surroundings. If that is what you are seeking. Hope this helps.

Did you end up painting the ceiling all the same? That's what I would've done!