Absolutely it can be done as hundreds of professional photographers over the years can tell you. There's a reason a good number of photography course teachers still nod approvingly when some student shows up with a K1000 in hand. It's because they know it's a stable, workhorse of a camera, perfect for learning with. yeah, they're happy to see others too, but I've actually known several photography teachers who go out of their way to purchase K1000's for their students as they can. Why? Because they've used either a K1000 or some other camera for years professionally and they know that they can reply upon them to do the job.
For the record I know three different photographers all in different fields who do shoot with Pentax regularly as well as Nikon and Canon when the situation demands it. They use the best camera and lenses for the job, the one they prefer and they don't worry about which lettering is on them.
One of those guys has been shooting with a film Pentax since he was a soldier in Vietnam and he was given his first one. Another has worked in fashion and advertising for more than 40 years and his digital camera of choice is a Pentax DSLR, of late, the K-7. For his own fun he also shoots with a grand assortment of vintage Pentax (and other brands) cameras, everything from an old SPII to a K1000. The 3rd guy, well, he's just not much into the "digital revolution" so far. He's still shoots several Pentax film cameras every day on the job even though it means he has to go and develop film and scan his negatives to use them.
I have nothing against other brands of cameras. I like several, but having used it for a bit now I wouldn't trade my *ist for a brand new Nikon, seriously. I just like this camera and if this is what using all Pentax DSLR's is like? Well, a year or two down the road when I am doing my thing professionally? I will be shooting a weather-proof Pentax wonder camera like the K5/K7. Why? Because every Pentax camera I've ever picked up and shot has made a huge fan out of me.
(Well, except for the P3 series. I did pick up one of those once. The design made me shudder though and put it right back down. It was so ugly I couldn't bring myself to further the acquaintance even though it's supposed to be a good camera internally.)
I do agree that even the most advanced Pentax cameras don't do everything every pro might want, but I think for the vast majority of people not into shooting for the NFL, for instance there is a heck of a lot of capacity there to utilize. Honestly, the K5/K7 will probably be more than enough camera for my needs, but that's me, and I don't happen to need to be able to shoot the Superbowl. If you do? Grab a Nikon and be happy. I'll stick with my Pentax though and I won't give a D- if the rest of the photography work thinks I'm silly for choosing the best camera for ME.
That goes for the color thing too. If some people want to take me less seriously because I am shooting a brightly colored camera? Fine, so be it. They'll soon shut up soon enough when they see what I can do with it, grin. Life is too short to be like everyone else and besides I'm just not much of a herd animal kind of person.