Both of the above answers are irrelevant, especially the one by pip.
First of all, "they" don't add a red dye to give that cut of beef that "lovely red colour." If you've seen a hanging carcass of a beef animal, the meat retains its red colour throughout the hanging process; it does not get that grey colour. The only time it will get that it MIGHT get that grey colour is if it's exposed to air and/or natural sunlight. Otherwise, all the beef that I've seen from a steer that was raised on this farm and slaughtered locally and hung to cure properly, still retains that natural red colour that makes it red meat. But colour is, in fact an indicator of quality. If the meat is a dark red colour instead of a lighter, more "normal" red, that is an indication that the animal, before slaughter, was under a lot of stress. Stress releases adrenalin hormones, which initiate the flight-or-fight response, and causes more blood to be released into the muscles than in other organs, hence the darker red colour. Dark red meat is tougher and stringer than meat from calmer beef cattle. Cattle that are calm and not in a high-stress mode before they are stunned and killed will have a lighter red colour (the more "normal" red), and the meat will be more tender.
Marbling is intramuscular fat found in between the muscle fibers of a cut of beef. The misconception of "the more marbling the better" is false, as a higher grade of beef seems to contribute to more health problems (because of the higher fat content) than leaner beef. Prime cuts of beef have a high level of intramuscular fat, which, on one hand, make cooking, broiling, or roasting them quicker and easier and makes them more tender and juicier, but on the other contains lots of saturated fatty acids and HDL's, not great for a person's health. A more moderately marbled cut of beef is what most consumers like to look for, one which not too much marbling but enough to still retain the tenderness and juiciness of a good slab of steak. Select or Choice is the grade that most supermarkets put on their shelves.