First you need to understand what types of light display can be done with your plain RGB type of device. Each fan has mounted in its frame LED's of three colours - Red, Green and Blue. All the Red LED's are connected together, all the Greens together, all the Blues together. The JRGB header can turn on or off any one colour or some combination of two or three colours. Without any ability to change the brightness of any one colour, that makes a total of seven colours plus none (all off) possible. I believe some headers (but NOT all mobos) are capable of changing the brightness of each colour, too, so more shades are possible. But note that at any one moment the entire fan light system can only display ONE colour. That is the limit of what a plain RGB system can do.
The other more advanced system called ADDR RGB is wired and controlled quite differently so that one lighting unit can display many different colours along its length. Thus more complex display like rainbows - either static or moving along the light strip - can be done. Mystic Light software can do that through the other JRAINBOW headers IF you have ARGB light devices, but you do not have those.