This is my very first backyard rink attempt, and I build it before I found this wonderful forum. Therefore I didn't put much thought in what color the liner should have, so I took the only one which was large enough at my local hardware store, which was green..

So.. should I flood the rink and get a new white liner before the water freezes (it's about 6C here in Oslo), or will the green liner suffice for this season?

If your rink is in the shade you're probably OK with a green liner. However if it is mainly in the sun then I would reccomend changing to a white liner if you still have that option. The darker color underneath will attract heat from the sun's rays and shorten the length of your rink in the spring. We have had this issue with dark colored leaves that blew in which I did not remove at the time.

Ok, thanks! I'll see if I get hold of a large white liner at reasonable price. If I don't, I'll just live with the fact that my skate season will be shortened. Fortunately (depending on how you look at it), we don't see so much of the sun during the winter time up here north. :)

Ryan is right, it's not ideal. The good news is that once you have ice, and you skate on it frequently, it will mar up your surface so that it appears white from above. The edges will still reflect the sun because of the liner draped over the boards, and you won't have quite as long a season, but it's not like it won't ever freeze. Think about the colors of lakes and ponds...they're dark brown and dark green, and they still freeze.