Medical Cross: I can see that the red of the cross may give reference to blood. I believe that it may also function as of means to catch someone's eye. For instance during combat in the good old days, a solider would be able to recognize a medic by the red cross on the helmet.
Coke: The red of the Coke logo could possibly correlate to joyfulness. As a child I have always had joyful experiences while having a coke in my hand. Its the same red in those polar bear christmas commercials. The only christmas commercials that I actually like.
IBM: I understand how the colors may work for this logo. IBM for years has been on the forefront of our technological advancements so intelligence would be a logical component of their blue.
GE has been around for ages and their blue represents the comfort that their products have to offer.
Batman: Batman is force to to be respected in the streets of Gotham city. His yellow displays the same confidence that it takes to stop evil villains in their tracks.
National geographic: Now here is a brand that displays learning as it teaches the world about itself through memorable photography and stories.
Cadbury: their products have been described as luxury according to my friends. And they have pretty good taste from my experience. The purple resembles that quality.
Sun mircosystems: Now here's a brand that is knowledgeable. Their purple resembles the intelligence that they have towards their products.
WWF: These guys make sure that all endangered species are protected and secure. It probably hints why they use black.
Playboy: This brand is somewhat of a classy brand. The black is the right touch for the intentions of the brand.