When multiple players need to make decisions at the same time, the active player makes their decision first, then the nonactive player. That's covered by these rule:
101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes any choices required, then the next player in turn order (usually the player seated to the active player’s left) makes any choices required, followed by the remaining nonactive players in turn order. Then the actions happen simultaneously. This rule is often referred to as the “Active Player, Nonactive Player (APNAP) order” rule.
Example: A card reads “Each player sacrifices a creature.” First, the active player chooses a creature he or she controls. Then each of the nonactive players, in turn order, chooses a creature he or she controls. Then all creatures chosen this way are sacrificed simultaneously.
101.4b A player knows the choices made by the previous players when he or she makes his or her choice, except as specified in 101.4a.
Since Player A is the active player, they'll have to choose a color for Iona, then Player B, knowing what color Player A has picked for Iona, will be able to select a color for their Servant.
Nathan Long
Wizards.Com Boards NetRep