If the sun is out all day, why is it not warm during an Arctic summer?

The primary reason is that the sun is low on the horizon all day. Thus, solar energy needs to pass through more atmosphere to get to the ground.

Explanation:

Hope this helps. :)

Most all of the visible light striking the snow or ice surface is reflected back without any particular preference for a single color. Most natural materials absorb some sunlight, which gives them their color. Clean snow, however, reflects most of the sunlight, creating a white appearance.

[ I commented on the other answer with the same little add-on, then I realised I could add an answer. Even though this is four days ago, meaning the person who asked the question doesn't need another answer, I hope to give someone else who might be looking at this some insight. ]