I want to be able to change the background of a cell colour if a second
condition is met - but for the background to remain unchanged if the first
condition is met.

So in the formula below, the background would be yellow if the 'c2'
condition is satisfied.

=if(c10,c1,c2)

My actual formulae are much more complex but if someone can point me in the
right direction in terms of principles that would be great