This discussion had me curious so I went and fiddled with the contrast controls of my Aristo VC Head. Cool little devil I can dial in contrast settings in 1/10th grade steps.
At the lowest contrast setting the light is blue, but moving through the range you can see the colour of the light change from blue, blue-ish green to green. So there is a mixing of light occuring from the two tubes. Never really paid attention to this as it just magically works.