Alot of drugs are considered 'me too' drugs. This means that they are the same medication, only altered just enough for the drug company to acquire a new patent after the old patent expires.
A good example of this is the drug prozac.
Released by Eli Lilly & co., it grosses roughly 2 billion in sales each year. When the prozac patent was nearing expiration, Eli Lilly released Sarafim, the EXACT same drug. Instead of being used as an antidepressant, sarafim was used to treat PMDD, or pre-menstrual dysphoric Disorder, which may not even exist.
Another example is Lipitor. Lipitor is the fourth incarnation of the original 'zocor.' All together, pfizer (the creator) is selling six different types of the same drug.
Zocor, Vytorin, Crestor, and Lipitor.
So pretty much the reason alot of these pills are the same colour is because they are the same medication under a different name. Sometimes big pharma will change the colour (as with Zanax) but most times the colour will stay the same.
Hope that helps.