Not finding anything about this ITT but it’s in the rulebook! Just wanted to call it out so it’s searchable here as well.
@hardy83 recently asked if Graveyard (or other building cards) would require paying the multicolor penalty of +1 gold if they are the first building you make (like I just did on P1T1 as [Necro]/Blood/Fire)
Answered my own q on page 12!
“if your heroes are more than one color,
the first tech building or add-on you build costs 1 extra gold.”