What does the dual color rune cost mean on units and spells?

Ruling: A dual color rune cost means you can pay that power cost with power of either color shown in the symbol. For example, a dual red/purple cost of 2 can be paid with 2 purple, 2 red, or 1 of each.

Nuances:

• If a card required both colors separately, it would show separate red and purple symbols rather than a combined dual-color symbol
• Single-power dual-domain costs (like 1 power that can be either color) exist and demonstrate this is the intended interpretation