I have two colour schemes. When I change Options/ contrast on one of them they are changed in anotherone as well. Is this by design?
That is not by design. In KDE Plasma, colour schemes are separate text files, typically stored under ~/.local/share/color-schemes/ or /usr/share/color-schemes/. Changing the contrast setting on one scheme should not change another scheme.
This is definitely not something I can do - my colour schemes are completely separate… I just verified this for myself by editing my ‘Magna-Tweak’ scheme, increased contrast to 100%, then save that as ‘Magna-Contrast’ I have two distinct schemes which have no effect on one another.
Instead of saying ‘one of them’ and ‘another one’ you should try being specific, otherwise nobody can actually replicate what you are doing… but certainly the ‘contrast’ setting in the option tab only affects the currently edited scheme.