There are the chemists and manufacturers who makes sure the proposed dyes don't destroy the medicine's active ingredients or create potential allergic reactions. There are also marketers who study the look of competing drugs, and show how their proposed pill will stand out from the rest. In addition, designers propose colors for a particular medicine, and help make sure there's no symbolic mistakes. All these people also talk about whether the medication will be a tablet or a capsule. Drug companies want pills to be as small as possible.
Larger dosages can be tricky, though, as the diameter of the pill can get large and look choke-able.
In those cases, the tablets take on a more oblong look. Eli-Lilly was among some big drug makers who didn't want to talk about the cosmetic marketing of particular drugs, though it's clear they aren't oblivious to the issue. They're marketing a new pill for women who suffer from premenstrual discomfort. The pill is exactly the same medicine as Prozac, but they knew some women might not like the idea of being on a green-colored pill known to treat depression. So marketers put the new name, Sarafem, on the pill.