that question is a real classic, but it has a very clear and almost easy answer.
Defining an object's characteristics is decribed by the layer system in CR 613. In this case the important part is this:
613.1e Layer 5: Color-changing effects are applied.
613.2 Within layers 1-6, apply effects from characteristic-defining abilities first (see 604.3), then all other effects in timestamp order (see 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See 613.7.)
And the definition of characteristic-defining abilities
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object's colors, subtypes, power, or toughness; (2) it is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect; (3) it does not directly affect the characteristics of any other objects; (4) it is not an ability that an object grants to itself; and (5) it does not set the values of such characteristics only if certain conditions are met.
"Ghostfire is colorless" is a characteristic-defining ability, so it is applied before any other effects, including painter's servant.
Conclusion: The Servant-Iona combo protects you from Ghostfire