I have written program that converts all screens into negative and vice versa positively.
For all 3 CPC's 464, 664 and 6128 compalated with the BASIC compiler 464, 664, 6128.
As you can see, twice as fast as in the BASIC.
You load the Bild.Bin &C000 (49152) and the program starts with CALL &7631 (30257).
The image is now converted to negative in the MC code.
With the key the Bild.Bin is then saved as Bild.Neg.