You wouldn't be wrong in BrE, Veli. When an adverb+adjective makes a compound adjective, the use of a hyphen is generally accepted (though not obligatory) if the adjective is attributive, but not if it's predicative. See this BBC article. The tendency these days is not to use the hyphen, because it's usually unnecessary, but I've never seen a BrE source that says it's actually wrong.
I Googled "colorfully tiled swimming pools" [images] and the most wildly colorful one was just variations on blue. I don't think I would call variations on one color "colorfully tiled". It could be an oddity but I did not find any of these oddities when I looked.
Here is the most "colorful" one I found, an interesting mosaic, but all in shades and tints of blue:
The swimming pool I have in mind is actually black and white for the time being... It's a task for kids where they get black and white pictures to color according to some vague instructions, e.g. find a coin on the bottom of this colorfully tiled swimming pool. So it's an imaginary pool.