Well, the human eye can see many color shades, including varients of greens, blues, yellows, and just about any other color under the rainbow sky.
Interestingly, the color of green is actually a combination of the colors blue and yellow, forming the tinge that you see before your eyes. Green is also considered a cool color.
You get a shade of green when you add black to the basic green color. If you add white instead, you get a tint of green.
So to answer your question, a shade of green is when you add black to the color green. it's usually a cool dark color.
Of course, if you're referring to window shades in your house, then I can only assume that you have shades on your windows that are a reflection of green color.
There's certain things that I would never want to see green. I mean, some people have green beer around St. Patrick's Day, but you'll never see me drinking any of that.
Speaking of which, it's about that time. Miller time, that is.