Just use the three FETs connected correctly and if you want to dim the overall colour just do it in software.
One way is to convert the RGB value into HSV coordinates reduce the V component and convert back into RGB and apply to the FETs.
Or you could try top switching the 12V power, but be aware you will get beating between the two PWM signals resulting in what looks like random flashes.
I am not sure how that video is supposed to relevant to your question.