Coverage, Cost, Color
Of the four major players, without even seeing their logo or hearing a voice-over, each one has built up enough equity and public awareness of their brand by sending a signal through use of distinct color. They can catch us at a glimpse, even through the peripheral flashes of a passing billboard or commercial. In the graphic above, the color cues are strong enough with these brands that having the wrong name on each color block doesn't affect our gut response that "red" is Verizon, "blue" is AT&T, and so on. Interestingly enough, each brands color seems to be appropriate (whether they had the choice or not): AT&T's blue draws on it's long-standing corporate history. Verizon's red also reflects corporate, but more aggressive. Sprint's yellow in-part comes about through a merger with Nextel, but still suggests speed. T-Mobile's magenta is the more unique of the bunch and suggests of it's European roots. (By the way, T-Mobile actually owns the trademark to the color magenta in the telecom space. More details here.) They do separate themselves enough from the other as competitors.
Could one of the major players change their color to green?
They certainly could, but at what cost. Equity is hard to get, and instant recognition of a brand through just the use of color (and nothing else) is perhaps even harder. Sure, Verizon could create buzz around themselves by ditching red and switching to green for their primary corporate scheme, but they'd be giving up a lot. The only time you could really justify a color rebrand of such a large caliber is if there has been a massive overhaul of how things operate internally in the company. Thus, this could signal that a new era has begun. I'm talking about "old things have passed away" type of era; not an eco-friendly, "green-initiative" ad campaign.
So is there room for an up-and-coming brand to break in? What colors are left?
The color staring us all in the face here is the already discussed green. It is clearly different from the brand colors of the major players and would have room to fit into the competitive color landscape just fine. Actually, two of the trailing mobile brands behind the four major ones are Boost Mobile and Cricket. Boost uses orange while Cricket uses green. Graphically speaking, without having to abandon ship because of a conflicting color, both of these have the opportunity to carve their own niche in our minds for brand recognition if they could ever make it into that top-tier of players. If one of them wanted to change color, at the level they're at in the marketplace, they could have some wiggle room to do so. Boost becomes green for a fresh look; Cricket goes orange for a modern appearance. But if they would ever change, they can't do it on a whim. You must survey the competition, who you are as a brand, and the significance of what a color represents (cause you're gonna need to live with it for a while). Oooh, what about gold? Probably not, but you can't say it wouldn't make you stand out. You just have to accept the perceptions and assumptions a color like that would likely carry.