| SlimGauge |
From the PRD entry for Darkvision, but emphasis mine.
Darkvision
Darkvision is the extraordinary ability to see with no light source at all, out to a range specified for the creature. Darkvision is black-and-white only (colors cannot be discerned). It does not allow characters to see anything that they could not see otherwise—invisible objects are still invisible, and illusions are still visible as what they seem to be. Likewise, darkvision subjects a creature to gaze attacks normally. The presence of light does not spoil darkvision.
You can see the illusion as what it seems to be. If the illusion is intended to seem to be 99 red balloons, when viewed with darkvision, it seems to be 99 balloons of indeterminate color.
EDIT: fixed quote tags
| Claxon |
If viewed as though it had color, it would immediately alert the possessor of darkvision that something was wrong (since they normally can't see color while using darkvision).
Certainly darkvision doesn't automatically invalidate illusions, so the only reasonable understanding is as SlimGuage said. You see balloons, of an unknown color. a woman singing in German.