Rules Compendium's section on spontaneous casters unifies the spells known rules enough for me to be comfortable ruling that you get the domain as spells known the same way you get all the other spells on your list.

The strictest reading would actually be that warmages learn spells when they gain access to a new level of spells. Thus, when you gain the Good domain as a Warmage 6/Rainbow Servant 1, you don't get any spells from it until soon after, when you gain access to 4ths, at which point you learn all the 4th level spells on the warmage list, and, since she "may select a domain spell to add to her spells known whenever she would have an option to choose a new known spell," this is her opportunity, so she could pick any one spell of 4th level or lower from the domain to learn.

Meanwhile, the 10th level ability allows you to "learn and cast spells from the cleric list, even if they don’t appear on the lists of any spellcasting class you have." So, normally, you only learn warmage spells when you gain access to a new spell level. This feature allows you to also learn cleric spells, even though they're not on the warmage list, but doesn't otherwise change the way you gain spells, so you would be able to learn all cleric spells of that level and only that level, because as a warmage, your spell knowledge isn't retroactive. Effectively, the strict reading gives you cleric 8ths and 9ths only.

I'm being facetious here, of course. The real answer is that this is an ambiguity in the rules, so it falls into the purview of page 6 of the DMG.
Quote Originally Posted by DMG
Often a situation will arise that isn’t explicitly covered by the rules. In such a situation, you need to provide guidance as to how it should be resolved. When you come upon a situation that the rules don’t seem to cover, consider the following courses of action.
  • Look to any similar situation that is covered in a rulebook. Try to extrapolate from what you see presented there and apply it to the current circumstance.
There's a very obvious comparison here—all the traditional arcane classes learn granted domain spells (and cleric spells) the same way they learn all their other spells. So, following this extremely obvious precedent, RAW's guidelines lead us to the most commonly accepted ruling: warmages gain all the spells as spells known automatically. 💁