Select the cells in the range A1:A50 and then click on Conditional Formatting | New Rule | Use a Formula ..., then put these formulae in:

=AND($B$1<>"",$C$1<>"",AND(A1>$B$1,A1<$C$1))

for condition (a) and this one for condition (b):

=OR(AND($B$1<>"",A1<$B$1),AND($C$1<>"",A1<$C$1))

For each one in turn, click the format button, then the Fill tab and choose your colour. OK your way out.

Hope this helps.

Pete