Having a monochrome playfield on ZX Spectrum is a common way to have no colour clashes - simply because there are no colours to clash! Also, most of those games then had masked sprites. This meant that each set of sprites had another "sprite set" for the mask, being actually 2x1bpp (bit per pixel) sets of data.
Some games coders actually used this to get the Sprites coded in 2bpp and used one of the 4 inks in mode1 to be the mask. This then took up almsot no more RAM used by data than the original(concerning masked Graphics). As a result some of those speccy ports have sprites in another colour than the background, which is actually better in terms of playability/look.
An example of this is '''Super wonder boy''' which could have even used one more colour for the sprites, but didn't.
*Super wonderboy (wonderboy2) : poor port... the sprites use only 2 colours while they could actually use 3 with no extra effort but some graphical job.
Yet compaired to spectrum version, the CPC version manage to have the sprite more visible thanks to their different ink.
==Masked Backgrounds==