i think i already expleined this in a previous post, but i will do it again.
When you use the strobe global, it push the dimmer and the strob value to full, but for the RGB system it doesn't effect the value of the rgb dimmers. This way you can have the strobe the color you want! So usualy you do a color cuelist whitch will set the color of your fixtures, and so when you will use the strobe global it will strobe with the active color of your cuelist!
Now if you want only a white strobe, you will have to creat a cuelist assigned to a playback button whith the strob, dimmer and RGB alues recorded...