If she hasn’t always been gray and you’re noticing now that she’s a bit of a grayish-green, there’s a possibility that she’s very close to molting, actually. For insect pokémon, molting can be a long process that can take months (as they’re larger than most things that molt and thus require a lot of energy to do so), and in some cases, the process can change a pokémon’s color slightly as the exoskeleton begins to separate from its body. If this is the case (and you’ll know if you can recall a time when your charjabug was more vibrant and maybe a bit perkier), then your next step is simply to make sure she has plenty of food, water, and soft bedding and … wait. It’s likely that Bubble will begin breaking free from her exoskeleton within a week.
If she hasn’t always been gray and you’re noticing now that she’s a bit of a cross between gray, green, and blue, then it’s also possible that she was exposed to a source of high magnetism or electricity and is attempting to evolve. If this is the case, you’ll need to make sure she actually wants to evolve. If yes, hook her up to a source of electricity (it won’t hurt her, I assure you) and allow her to draw as much as she needs to complete the process. If no, then you’ll likely have nothing to worry about, but keep her away from high-powered magnets and indoors during thunderstorms.
Finally, if she’s always been that color, there’s a possibility that it’s simply genetic. Off-colored pokémon are rare but normal, and it may be that you’re lucky enough to own something on a far end of the color spectrum for what a charjabug looks like. Congratulations, anonymous!
Regardless of the cause, rest assured that Bubble is indeed very likely happy and perfectly healthy.