Spectrum is only an issue if you don't offer enough blue. And most led fixtures do a 50:50 split between white and blue (or at least close to 50:50). If you run both channels at 50% power, you are probably getting enough blue although the tank will look quite white. You'll find that it only takes a small amount of white light to 'wash out' a much bigger volume of blue to our eyes. But that doesn't mean the blue isn't there and still doing it's job helping the coral feed itself and grow. Even at 100% power to the blue channel it only takes 10% or a little more of the white channel to make the tank look white. And as you add more white it may go from a cold white to a warmer white depending on which white leds you have in your fixture.
What shade of white you run is more about what looks good to your eye than what's best for the corals. They need blue for the zooxanthellae to do photosynthesis. They do use other spectrums to create pigments. Some pigments will use the high energy blue spectrum to create (fluoresce) different colors than the coral is under white light. So some people like much more blue to even just blue which makes some tanks look like a 1970's hippie poster under a blacklight (if you are old enough to remember what they were like).
Red and green help coral's color to look more natural, i.e. they have red and green to reflect and show their natural colors when your tank is white. You don't need much, some would even say you don't need any. There is some red and green spectrum in the white leds.
Some people make a huge issue about the color of their tank as they see it, I'm not that picky. Some other people make a big deal out of the exact spectrum they get for keeping corals healthy and growing, most currently sold fixtures do an adequate job here as well. Some people will say you need a high quality (i.e. expensive fixture) and others are on tight budgets and go for the cheap fixtures. In my experience, the cheap fixtures are able to do 95% of what the expensive fixtures do as far as the spectrum is concerned and do it for a lot less money. Then it's a matter of what kind of 'special' features and controls you want, they cost extra as well. Dimmer knobs and add on timers still do a perfectly acceptable job.
