Ultimately I say get what you want. Get the best camera for you in the color you'd like. Life is too short to give a rat's arse about what other people think about the color of your camera. If they're shallow enough to care more about what color (or brand) your camera is than what it's capable of and what you can do with it then they're probably not people you want to do business with anyway.
They're not seeing YOU. They're not seeing your work, they're only looking for what they believe to be cool, trendy and professional, what looks right to them, not what necessarily is. That's utter BS, IMHO.
Technically speaking a green K-x is the equal of a black K-x and and K-x of any kind is more than enough camera that you should be able to take it to any event and with a little skill on your part get the job done. Yeah, having a K-7 or a K-5 is probably going to be a better choice if you're going to be doing pro work, particularly outside, but a K-x isn't a bad choice for someone who is doing some pro work, some hobby and who needs an entry level, less expensive model to start with.
I'm probably going to start doing some pro work later this year or maybe early next year with one myself. It just so happens that it's likely that one I'm going to be getting will be a black camera, but I wouldn't just sneer at it and not buy it and use it if it had happened to be pink or whatever. If I was the type to be that conscious of what other people think I probably wouldn't be using a Pentax in the first place. But I do, and with pride, and I would still say that even if my Pentax DSLR was multi-colored in primaries like that one that looks like a plastic lego toy! It's what's inside the camera body that ultimately counts. I do like a nice looking camera, sure, but function always trumps fashion in the end.
Pentax actually having the option for both? Win-win scenario I think. Me, I wouldn't actually mind a K-x or a K-r in Schiaparelli shocking pink, or Halloween Green or Crayola Purple. I'm an artist and I'm a definite freak for color. Black is chic, but it's not the only "cool" color on the planet. IMHO a camera is only as "professional" as the person using it regardless of what shade the plastic cover is.
Forget the pack mentality thing and get what makes you happy. You'll like your camera far more and probably use it more for having one in a color you actually like. You're the one who has to live with it after all. It should really be about you. Never mind about what the rest of the world thinks.
We live in a world where people routinely elect lying idiots to higher office. Where people who already have a cell phone wait for hours in the pouring rain in a bad economy to get the latest and greatest I-phone even though it probably costs as much to 1/2 to 3/4 of a month's rent. Where people who are barely making a paycheck still pop $5-6 for a cup of coffee several times a day when they could just buy a darned coffee maker and some "gourmet" coffee and make that same cup for .75 back at the office.
I mean think about it. These are the very same people who are going to be sitting there, giving your maybe not black K-x a dim look and deciding how "professional" you are based on the fact that your camera isn't the black Canikon they expected you to have. Your potential clients. Now, which seems more silly to you? Actually getting a green K-x or letting people with that mindset decide what kind of photographer you are based on what brand your camera is and what color it is?
Yeah, you might lose a job here or there buy buying a colored K-x. But I'm quite sure that in the end it won't seem nearly as bad as having to deal with people who probably haven't a clue as to what a good photographer's camera kit should really entail. People who care about the color of your camera more than the contents of your portfolio are very likely to drive you crazy doing jobs for them. On the other hand people who get why you bought a non-standard camera are likely to be fun to work with. They might actually like it when you get creative and welcome something more than your standard boring one note portrait session.