Streaking is the worst problem with a white asphalt shingle roof. You may get it worse than average because of the shade factor you have identified.
White roofs get plenty warm in the sun, probably not as hot as fast as a black roof, but it will dry satisfactorily in the sun.
Casey

You really should look at some white roofs before deciding. They aren't the most attractive look with most houses. It takes the right house to pull it off. Not worth the 10% "incentive" to me to make my house look wrong.

With proper attic insulation (R60) and ventilation such as a ridgevent and vented soffits, even a black roof will not radiate heat into your home via the attic. It won't be shorter lived either. Put your money into insulation and venting. And replace the whole roof, not just half. You'll never get the shingles to match otherwise.

The tax credit is for asphalt roofing shingles that have reflective "cooling granules" and meet Energy Star requirements. Since the infra-red light reflectance of the granules is largely independent of the color, it is not necessary to use white granules. Typical colors vary from light gray to medium dark brown.

A light color and/or an Energy Star rating does not necessarily mean that a material qualifies for a tax credit so you will need to get a "Manufacturer Certification Statement" to be sure.

Try CertainTeed Landmark Solaris asphalt shingles.
http://www.certainteed.com/products/roofing/residential/designer/317920
http://www.certainteed.com/energytaxcredit/

IR reflective roofing materials will provide the most benefit in hot sunny climates where air conditioning is more critical for comfort than heating. That is not the case in New England and your roof is shaded so I find it strange that there would be a net energy savings or that a tax credit would be offered. If the additional cost of the shingles is greater than the tax credit, I can see no reason to consider reflective shingles.

There will certainly be no change in the heat conducted through the roof structure from inside to outside and therefore no effect on roof ice dams.

White roofs are so easily stained that they usually contain additives to inhibit biological growth. Light colored shingles will usually last a bit longer than dark colored shingles in a sunny climate.

Here is a link that might be useful: Energy Star list

i tried to find white singles and the closest was shasta white, and still has a grey overtone...we had a shade darker grey and yes it does show streaking, and don't know if i'd go that route again. but our house is half brick and alum siding painted a med grey...

it's interesting in your area, "white" is what they classify a cedar shingle, too...

I would assume that the white roof credit is due to it's albedo effect on global climate change. The energy the sun sends down is one type of wavelength and that is reflected back up to the atmosphere and does not interact with Green House Gasses. But once heat is absorbed it is changed to the wavelength on the energy spectrum that DOES interact with Green House Gasses. (dark roofs absorb that energy and changes the energy wave length when it releases the heat). So the tax credit is so that people will make pockets of light colored areas that reflect rather than absorb the suns rays.