ever seen roots growing above ground? the light root prunes them...you will get a quicker,fuller root mass from letting them go out to the edges...
Gotta say I have not seen proof of this in my gardens. I've done the 16 or 18 ounce solo's clear and double cupped red solos.
No difference really. I now use clear cups, algae and all. Works for me might not be best but I like to see root growth.
I'm sure this is not the preferred way to grow plants but if you make them hearty and veg for 60 days or so you can have some really nice plants.
I've grown in these 7 ounce cups until the stem was bigger than a pencil. Clear ones at that. Just gotta water...
I do agree though, darker is better where roots are concered.