There is a lot of misunderstanding about the Kelvin color temperature scale, so here is a quick overview of the main problem of using the scale in isolation when comparing different lights.
By itself, the Kelvin temperature scale is a poor method for comparing different lights, since the number itself doesn’t tell you anything about the color. Most people think the higher the number...the more blue the light will look. As you will see below, this is not always correct. Take a look at the below chart, which was created by the CIE in 1931. It shows the various color combinations of red, green and blue light. The horizontal sloping line is called the blackbody curve, which is the area where the 3 primary colors overlap and create ‘white’ light. The numbers on the chart are the Kelvin temperatures we use to describe white lights. A very important note: the Kelvin temperature is not a specific color. As an example, I highlighted the 6000 Kelvin line on the chart below. Notice that it is a line that extends from the green/yellow region down to the pink region. This means you could have 10 different 6000K lights, which are all technically 6000K lights, but they all look completely different over an aquarium. That is why the Kelvin scale is a poor comparison system…by itself. Ideally, you want to know where on the chart the color spectrum falls (by using x/y coordinates), and this is dictated by the spectrum of the light.