While spectrum is not totally unimportant, basically as said, anything between about 3,000k and 10,000k will grow plants. Intensity/par is much more important.
Remember that the K number is ambiguous by nature. Not only will a bulb put out light in other spectrum's than what the bulb is called, it's really used to describe a visual color (as far as I remember). It's much more of a description of what you should expect to see, rather than a scientific representation. Many bulbs will come with a graph but that really doesn't matter in practice either.
Making it even more ambiguous, many of us have preferences as well. For example, I love a 10,000/6,7000k combo on a green tank. The more reds in the tank, the further down the scale I want to go. My current tank has a 6,7000/Rossette combo. The overall look is more similar to incandescent/warm white light bulb than it is to a true white florescent tube. You could have never made me believe this combo would look good, and it took some time getting used to but my reds are much brighter than a 10,000/6,7000k setup.
Further adding to confusion. Once you have more than two bulbs, you may end up liking a purple or red bulb in the mix that would look absolutely horrible on it's own. They are often not as bright looking as a more typical 6,700k bulb, even if they put out the same PAR/intensity. Therefore you can get away with it because though it's not appealing on it's own, and seems very dim, it can often complement other bulbs to make your plants look different (and hopefully better).
