All auras have colors, but they define the creature's emotions only
This is described in the same book, under the Read Aura (Perception) skill unlock:
Read Emotion Aura: The colors within the target’s aura reveal its emotional state.
And under the spell Analyze Aura:
You peer into the aura of one target creature or object, gaining valuable information about its condition and nature. Each round, choose one of the target’s four auras. This spell functions similarly to the read aura occult skill unlock, but doesn’t require checks and returns results on all the target’s auras in an instant.
Alignment Aura: You study the target’s spiritual nature to determine its alignment. You also learn the type and power of its alignment aura, as detect evil.
Emotion Aura: The colors playing within the target’s aura reveal its emotional state, granting valuable insight into its psyche. You learn a general summary of the target’s current disposition, as well as its attitude toward any other creatures within 30 feet of it. For the duration of the spell and for 1 hour afterward, you gain a +2 circumstance bonus on Bluff, Diplomacy, Intimidate, and Sense Motive checks you attempt against the target. Objects do not have emotion auras, except intelligent weapons and similar sentient oddities.
Health Aura: The flow of vital force animating the target becomes plainly visible, giving you insight into its physical condition. You know whether the target is unharmed or wounded, Whether it is poisoned or diseased, and whether it is under any of the following conditions: confused, disabled, dying, nauseated, panicked, staggered, stunned, or unconscious. Further, your insight into the overall strength of the target’s vital force reveals the total number of points available in its ki pool, grit pool, or similar resource. Objects and most undead creatures don’t have health auras.
Magic Aura: You determine the number and power of all magical auras on the target (as detect magic to determine a magic aura’s power), as well as the school of each aura. You can attempt Knowledge (arcana) or Spellcraft skill checks to determine the school or identify properties of a magic item, as normal. If cast on an item, analyze aura cuts through the obfuscation of the magic aura spell.
That said, each aspect of a creature's aura reveals something different about it, if it is vibrant or fading, if it has animations or is quiet, the forms it projects, and finally, its colors define their emotional state.
With the exception of magic objects
As also noted under the skill unlock:
Objects typically have only magic auras, though some also have alignment auras (and intelligent items have emotion auras).
As for the visual of magical auras, they have never been described in any official books that I know about. We know that spellcasting creates obvious special effects that everybody knows that a spell is being cast, and has been depicted as magical runes around the hands of the caster, or around the target of the spell. Other than that, all seems to indicate that the developers left it open for creative interpretation.
As such, that is how I describe magical auras on targets, they have magical glowing runes around the target that linger while the spell is in effect. And as GM, that is how it makes sense to me that magic auras are so easily identified even by novice spellcasters, even if they can't identify the nature of the magical aura (the spell's school).