The attribute "ram" define the background colour (where a bit is 0), a foreground colour (where a bit is 1), if the dark variant of the colours should be used, or if the colours should flash at a fixed rate.
If the background is a single colour, the sprites can be ORed on. If shading is to be used a mask is often stored with the pixels. The mask is used to remove or retain (depending on the mask type) pixels on the screen. The mask is used final result on the screen to remove pixels, is the sprite pixels are result of the mask then put over the topsprite pixels.
On the Amstrad, the number of bits to define a pixel, the number of colours that can be used, and the width of the pixels is different for each mode.