It looks so much better at night, its almost as if its a completely different castle. It actually looks kind of strange during the day when the sun is hitting it directly, it makes the blues and pinks look way too bright.
As much as I love the night lighting on the castle, I'm going to have to go with daytime. If you catch it at just the right time of day (usually late afternoon, or sometimes very early morning)...well its a very beautiful sight. I know I have a picture of the lighting I'm talking about...just a matter of finding it. I really need to tag my pictures better.
For me its definitely during the day, it feels bigger and more real during the day. At night it just really seems more cartoony to me. There is nothing better than that first glimps of the castle when I walk out from the tunnel after arriving at DL.