Colorful eggs are literally easter eggs. They appear during the easter season in the dungeon. Eating a colorful egg satiates a tiny bit (about 30 calories) and yields a surprise item.
These eggs look different from the usual eggs and they contain random items. The BUC state of colorful eggs does not apply to the items within — a cursed colorful egg may contain uncursed or even blessed items.
Identification reveals the item that is hidden inside, e.g. an uncursed sky blue potion egg.