* The colours of each 8x8 "attribute" cell is defined by a block of ram following the Spectrum's bitmapped screen, each byte represents one cell and each byte defines paper colour, pen colour, flash enabled and bright enabled. The colours for the pens on the Amstrad are defined by writing to the Gate-Array's palette I/O registers. The pens are read from the pixel data and the resulting colour is looked up in the palette registers.
*The Spectrum can display all 15 colours on the screen. *The Amstrad can only do the same in mode 0, but this has wider pixels (approx 2x1 ratio). If the CPC's mode 1 resolution is chosen, it is not possible because only 4 colours can be chosen. Anyway some raster interrupt tecniques enable to mix (with horizontal limits) different modes or even to change the palette so more than the theoricall inks number can be displayed on the full screen. Such way was actually often used in many Speccy ports. While the screen displays 6 colours, the actual games window is still monocolour (2 colours... generally Black+one other ink...) or "almost monocolour (only background being monocolour and sprites having 3 inks... or some inks different from background's tiles...) typical examples are Strider (monocolour tiles and 3 inked sprrites), SuperWonderboy (Monocolour Background and monocolour sprites yet differently inked), PacMania or Black Tiger (monocolour game's window/playfield yet multicoloured HUD...) and so on (see later in this page for more examples).
*Normally Spectrum graphics is stored in 2 colours, which means 8 pixels for each byte. In Amstrad mode 1, each byte defines 4 pixels. So for the same graphics you often need twice the RAM on the Amstrad. (This is a case where graphics without transparency are used). If transparency is used, then the amount of data can be the same.