if you only want red and blue when you press auto create, have two colors WITH the chekck boxes next to their color AND without the RED check box , the other 4 colors have No check boxes (they are not selected to be used by effect) AND RED Box those 4. RED means to not allow the value to be changed. so if you have the last 4 colors unchecked and you put the RED (dont ever change this setting)
you end up with two colors being randomly picked
Color1
Color 2
Color 1,2