Tough one. I am an October, but I sort of envy you. One, I think that summer and winter lend themselves to the most vibrant colors. Two, you will have instant inspiration.
Since you need to plan both, I would actually try to find a color scheme that fits both. While you can always edit and alter after you set a date, it would be so confusing to have two completely different styles in your brain as you research.
As long as you are not set on a Christmas style wedding if held in December, I imagine that you could actually choose very similar pallets, with different inspiration. For example, jewel tones fit both winter and summer. In the winter, you would take inspiration from something like fun indoor lighting, while in the summer flowers and birds would be a better inspiration.
Other color schemes that could do both summer and winter:
– anything travel related. Lets say you guys love Paris. A sophisticated and muted color theme, such as grays, blacks, and whites, would be reminiscent of the Eiffel Tower year around. You would just reflect the season with your choice of accent color, such as pink in the summer, or really even pink in the winter (just use different flowers/details).
– anything era related. Some color ideas/themes have no season (an art deco wedding would use the same colors all season). Others would only change with the details. If you want Hollywood 50’s glam, golds, blacks, and red, for example, can go winter or summer, you would just play them differently (winter I would think chandeliers and plunging black Bridesmaid or Best Man dresses, while summer I would think champagne and sunglasses).
– a carefully thought out nature theme could even share colors. If you did white and green, that would be gorgeous all year round, you would just need to alter how you achieve it.