A fair question Dave P
Sadly colours of the helix cannot be seen visually no matter the scope size, as its not a matter of size but surface brightness. The total brightness of the Helix is quite bright, but because its so large, its surface brightness is low, not enough to activate the cells of the eye retina that sense colours. Bigger scopes show a larger image but not of higher surface brightness. Going to lowest power eyepieces makes it brighter but only to when the exit pupil from the eyepiece is smaller than the iris of the dark adapted eye. Going lower means a bigger exit pupil, some light will be lost, so the image will not appear brighter, just smaller.
This explains the exit pupil concept
http://www.singularsci.com/Accessories2.htm
Scott