What is the timing of "you may pay" abilities?
112.2c Each paragraph break in a card’s text marks a separate ability.
  
  603.1. Triggered abilities begin with the word “when,” “whenever,” or “at.”


Dromar's triggered ability is all one ability. It resolves all at once. "If you do" is not a separate trigger phrase.


Dromar's ability triggers.
Dromar's ability resolves. Follow the instructions of the ability in order:


You may pay [cost]. You may also activate mana abilities at this time.
If you do, choose a color.
Return all creatures of that color to their owners' hands. (If you didn't choose a color because you didn't pay the cost, no creatures match.)




  116.1d A player may activate a mana ability whenever an effect asks for a mana payment (even in the middle of resolving an ability). No player gets priority in the middle of a resolving ability.


Archangel's static ability functions when you declare attackers or blockers.  In other words, you declare and pay at the same time.


You attack with X creatures and pay {X}.


No player gets priority in between when you declare and when you pay. This is covered by rules 508.1 through 508.3, which are too lengthy to quote, but you can find them in the Comprehensive Rules if you want a more detailed explanation.