You want something in the 5000-7000K range, for freshwater. Usually people cite 6500 or 6700K as the favored type, though some people say 10000K is also good. However, I have also seen people say that 10000K tends to make algae problems, but nobody seems to have problems with 6500K.
The "color" is basically how our eyes see the kelvin rating. "Cooler" bulbs (lower Kelvin) are reddish and "hotter" bulbs are blueish, up to purple. Think "infrared" (below red) and "ultraviolet" (above violet). If you look at that 18,000K bulb it looks faintly purple. It's a little misleading when you think of lights being "warm" or "cold", because that's actually backward from the Kelvin rating... usually we say a white is "warm" when it's yellowish/reddish and "cold" when it's blueish, but in reality, blue stars are literally hot and red stars are literally cold lol
The difference in the bulbs is the wavelengths and the spectrum of each light... blue waves have shorter wavelengths (higher frequency) and can penetrate water better, which is why marine setups often use blue lights. In nature, by the time light has hit the reefs, they've gone through several feet of water and the only wavelengths left are the blue ones.
"Daylight" bulbs are usually around 6000K, and that's reflected in the color. So when you're picking "color" and I'm telling you to go by Kelvin, it's actually the same thing.
ETA that the colormax bulb doesn't seem to give you a Kelvin rating but I wouldn't go with it anyway...
comparing the spectra of 20,000K and 10,000K. Still energy at all the wavelengths (i.e., white light), but distributed differently
ETA again that this was probably more than you cared to read... sorry about that XD