The five colors are white, blue, black, red, and green. Conquerorâ€™s Flail canâ€™t give a creature more than +5/+5. (Gold, artifact, and colorless arenâ€™t colors.)
The first ability of Conquerorâ€™s Flail constantly updates as the number of colors among permanents you control changes. If the number of colors decreases and the damage that was marked on the equipped creature earlier in the turn is now lethal damage, that creature is destroyed. Similarly, if its toughness becomes 0, that creature is put into its ownerâ€™s graveyard.
If you move Conquerorâ€™s Flail from one creature to another, thereâ€™s no time in between that itâ€™s not attached to a creature. Assuming itâ€™s your turn, your opponents wonâ€™t be able to cast spells.