I have polygons with the area in the attribute table and I want change polygons to red color if they have area less 70.
Here's another method. It's not really any better than MrXsquared's answer, but I had already started writing it.
Use data-defined settings to control fill color
with this expression:
This expression will set the fill color to red (#ff0000 in hex notation) when the area is less than 70, and green otherwise.
If you want to use different shades of red and green, you can get the hex notation for the desired color from the "select color" dialog - just select the color you want and scroll down to find the hex notation near the bottom.