The yellow may only make an appearance if Altair is actively after David--that is, if Altair has a reason to tail him, or if Altair knows he has already done something to warrant assassination, like widespread mind control. Otherwise, he'd show up as whatever color suits his current personality/identity.

Caveat: If David happens to be sporting some sort of telepathic block or shield at the time, Altair may not be able to see him as ANY color, and assume he's a harmless civilian. 8O