By 2 colors it means two specific shades, so that could be black and white, or black and one shade of gray, or a shade of red and a shape of blue, etc. But never any more than 2 specific color values at a time.
Only two colors count, Green and black, Black and White,etc... no gradients or shading from the color.
Obviously when filters/shaders are applied to that restriction will be a lot more colors, but you need to put an On/Off button to activate/deactivate filters to chect fits with the colors rules.
If you need grey, you'll have to use dithering. I created a small checkerboard b&w tile and used that to create a grey background. The dithering will create visible patterns in the grey and might look a bit odd when webpage is zoomed (on my browser game)
