Two things you might want to consider, as I know I am for my future 'sunroom' build...
You will get loads of light, even in more northern latitudes, because even thought the sun is a little weaker, the water depth is also alot less for our tanks compared to nature. A 14-16" diameter sola-tube can easily match the output of a 400 watt halide in that case at noon... even here in WI. Now, when in doubt, always go larger, for sure.... you can always restrict the light that comes in, but you cant add more later as easily.
As for color, keep in mind that a very good reason to 'overshoot' your skylighting needs is that then you have some play as far as adding in color filters to filter & 'tint' parts of the light. It can be as simple as placing a piece of blue tinted acrylic or film in the skylight to filter some of the light coming through to bring back the blues. A checkerboard pattern of blue film in the skylight or above the tank somewhere should be all you need, and any light lost from all that 'filtering' of the light would be made up for by the fact that you have so much more of it to go around.
Of course, for 'after hours', some backup lights are an idea. I plan on a 4'x4' (or two skylights that add up to 65"x65"... havent decided yet) light shaft over a 6'x6' tank, with about 24x 80watt T5s as backup/supplimental lighting... 1/3 running blues and 1/3 running actinics so I can suppliment the daylight in weaker months, and 1/3 running aquablues/6500Ks so I can have a full spectrum at night. Chances are, I wont even need these many anyways... I know of 480g tanks (I plan on 540g) which are running on only 20x80 watt T5s, with SPS top to bottom.