As far as GTA Online glitches go, this is as harmless and as useful as they get. While the game always featured extensive car customization, which has since been significantly expanded via the ever-growing Benny's Original Motorworks introduced in the Lowriders DLC, bikes haven't really gotten a lot of love. In fact, the non-motorized ones have no customization options what so ever.
With the size of the GTA V community being what it is, it should surprise no-one that there are a ton of demographics within it with different in-game interests and tastes. There are players who focus entirely on stunting and parkour maps, some players roll with a full-on biker style going on. Others have their own taxi service, or go hiking frequently across the landscape of Los Santos and Blaine County.
Of course, there is also a bicycling subculture in GTA as well, though these people tend to fit into the stunting community as well. The vehicle of choice for these players goes on two wheels and needs some serious pedaling work to reach high speeds. Unfortunately, the bicycles in GTA Online are as you get them - no upgrades or customization.
There's no need to fret, however, as there is a method out there for making your BMX bikes less... dull. Granted, it does require the use of a glitch, so this might be patched (Please, Rockstar, don't!) sometime soon, so get your colorful BMXs while they last!
First, travel to the garage where you want your new BMX delivered to. You'll need to pull up your phone and go to the Pedal and Metal Cycles website. Here, you select the BMX, but do not proceed. Immediately hop over to the Legendary Motorsports website, choose a Z-Type with the desired color option. Go through the ordering process up to the point where you need to select a garage, but back out there and via the internet history option on your phone, select the BMX link.
All you have to do now is complete the ordering process and voila, you've got yourself a nice and colorful BMX bike. Luckily even if the glitch is patched, the color selections save permanently via the current glitch, so they won't be rolled back. This can also be repeated without putting the phone away for multiple, different colored BMXs.
With ideas flying around for what direction GTA Online's DLCs should go and considering how much you can mod a BMX in real life, that is yet another possible venue of expansion.
Would you like to see broader bike customization in GTA Online?