Comparative effectiveness of antibiotics against colonies and growing bacteria
It depends on the kind of antibiotic. Some antibiotic are bacteriostatic  i.e. they primarily stop the growth while some are bactericidal i.e. they kill the living bacterial cells. Some antibiotics can have mixed effects. Bacteriostatic antibiotics would not be very efficient in eliminating a colony.

Apart from the mechanism of action, there can be other factors that would dictate the efficacy of the antibiotic; these can all be considered under pharmacokinetics/pharmacodynamics:


Degradation/clearance rate of antibiotic
Rate of diffusion of antibiotic inside the bacterial colony (this rate would be less when the cells are inside a big colony compared to when they are disperse).
Community organization and dynamics inside the colony (like user137 said in their comment, certain biofilms can be more resistant to antibiotics)