In the Lighting page of the web application, there is a section for deciding custom colors of the RGB-units inside the PC:
There are three different ways of changing the color here. The first one is through the color slider and the two bars along the side:
The main 360 degree slider in the middle is what we call the color slider. This lets you pick the color value by either clicking somewhere on it or click and hold to drag to your prefered value.
The left bar is what we call the saturation slider. This lets you pick the intensity of the color chosen.
The right bar is the hue slider. The hue is where you choose the base color or the value chosen.
As these can be quite hard to use and get right, we also have the two options below it:
The first option lets you change the color by adding the hex color code. There are plenty of hex color code generators to find online to use and find your preferred color.
The second alternative is adding the RGB color. For the uninitiated, RGB stands for Red Green, and Blue. The LED-diodes inside of the RGB-fans or strip utilize these three colored lights to create a custom light option. Each color has a value between 0 and 255, where 0 is turned off and 255 is fully lit. On this scale above, R255 - G0 - B0 would be a red light. R0 - G255 - B0 would be a green light and so on.
As light from a monitor and light on the RGB-units can vary a bit due to light guides/lenses on the units or case colors, you might need to experiment a bit to get the colors you like. If you find a color similar to what you are after, you can adjust the specific RGB-values to try and match it. If you want to test it, start with R255 - G0 - B0 to get red. Mix in some green by doing R255 - G50 - B0 and you will get an orange color. Swap out the green to blue with R255 - G0 - B50 and you'll get a pink/purple color. See results below:
Chosen light may vary depending on fan model due to these usually having different solutions of how the light is displayed. The Momentum fans in the picture above has a light guide around it that makes the colors less "aggressive" than some other. To match different models of fans, you might need to experiment a bit with the color picker and possibly have slightly different codes for these.