The only way an AR is going to be a collector's item of value is if it is an original configuration or test/prototype model...at least until they become illegal but grandfathered. Even then, there are something like 15 million in circulation. For a current model, if you want to wait 70 years for a plain Jane rifle to be financially valuable, go for it.
Other than Colt requiring different sized pins from everyone else, it is still just another AR. There is nothing noteworthy about them. I do not see where only brand name will matter in this case. This is not like something like the HK P7 pistols, where they were not only different, but that they had characteristics that elevated them above the average comparable firearms of their type (not to mention a dedicated following which Colt ARs do not have in any meaningful way).
Other than Colt requiring different sized pins from everyone else, it is still just another AR. There is nothing noteworthy about them. I do not see where only brand name will matter in this case. This is not like something like the HK P7 pistols, where they were not only different, but that they had characteristics that elevated them above the average comparable firearms of their type (not to mention a dedicated following which Colt ARs do not have in any meaningful way).