Painting - Worst of prep work almost done

The ceiling has been washed down as well as most of the walls. Everything was disgustingly yellow from cigarette smoke. I am going to give everything a 2nd wash before I paint. The existing paint is semi-gloss, which on the plus side, washes quite well. I used Oxi-Clean even though it said, no reason given, that it wasn't for use on latex. It cleaned better than TSP and didn't do any damage. I would like to go with flat paint this time around. Any problems I should know about with flat over semi-gloss? I could step up to an eggshell. I just want to avoid the sheen which shows all imperfections. I was going to go with a medium green on one wall with the rest being in the beige family. Then I read in a post that green was not an easy color to paint with. I plan on using SWP. Any advice would be appreciated.

You may want to prime to seal in the tar/nicotine stains
They bleed through latex paint like you wouldn't believe
If it's not real bad, some latex Pro Block from SW should do it
(if it is real bad, you may need to use oil-based)
You should prime semi-gloss anyway, so it's not a real big deal
Do a light scuff sanding first to give the primer some "tooth"
You can use a flat no problem

I haven't found green that tough, but I like your idea anyway
Deep colors can be tough
But if you use that SW Deep Colors (sometimes called Deep Accents), it's usually not a problem

However, the lighter the paint, and flatter the sheen, the less imperfections show