"Your colour spray lights up the crypt for a moment. You've caught the horror cleanly in its fan… but it continues to advance as if the spray of blinding light didn't exist."
Players are there to experience a world and events, with their characters in the middle of it, solving problems by their wits and kicking tail with their characters' abilities. Describing that world in a way that they can engage with it directly, instead of abstractly via the rules, is vital for many groups to maintaining the roleplaying part of "roleplaying game". The suggested narration above, or something like it, helps your players stay in the head-space of your imaginary world. Players should not be told why something unusual happened, by default—there's not much fantasy and wonder in exploring a fantasy world if everything fantastic is explained right away!
The player will already be puzzling at the discrepancy: "Why didn't that work? Is it resistant to other spells? There's no light in this crypt… does it not need its eyes to see? The chance of it being undead is pretty good if so…" For most people, this is fun! Don't interfere with that fun by immediately handing players the answer to every unknown they run into.
Of course, there are things that won't be unknown to the characters, things that naturally call for giving the players more information.
If the character does have special knowledge, such as Knowledge: Religion (which covers undead's abilities and weaknesses) or Knowledge: [local area] (which might give relevant rumours of the creature's behaviour), then ask for a check and follow up a success with the sort of information that would give them: "The colour spray failed to affect it, and you know that undead are immune to spells that confuse the mind…" or "You remember a story once about a young man who'd disturbed its lair being followed by the creature through a blinding, deafening storm as if the weather wasn't there."
The only time you should just tell (instead of show) your players why something is happening with no justification in the game world (like Knowledge skills, or reminding them of the last time they encountered a similar creature) is if it would directly interfere with what your group thinks is fun about the game.
Some groups play for the tactics and the efficient combat-plays. Further, some of those groups won't be interested in puzzling out the nature of fantastic things through the clues in your narration. If you're playing in this kind of group, by all means lay as much information on the table as you've got and move on with the fun parts of the game.
Detect Magic would sense a Moderate or Strong Abjuration aura when looking at the Antimagic Field(AMF), but nothing within the field.
Despite being a sphere of antimagic, AMF is still a spell effect and produces an aura as anything would. Per Detect Magic, a 6th level spell would produce a Moderate aura, and its 8th level version produces a Strong aura.
Detect Magic's stated target is an area 60ft long, not a personal spell. Any range that extends within the AMF would be nullified. They would be unable to determine the source of the aura, unless they could see enough of the sphere to determine its center.
A further note, except in cases of GM Fiat, AMF will be centered on a creature. The spell states "An invisible barrier surrounds you and moves with you." Furthermore, there is no RAW item that has maintains an AMF, and the closest, the +1 Tower Shield Equalizer values the once per day use of AMF for 10 minutes at over 100,000g.
Best Answer
As written, I think it is unclear (bad wording as you say), what is meant to happen and it'll truly be up to the DM to decide. But I'll take a stab at it:
Let's break the spell description down a bit. As per the Spell Turning description:
This DM would rule that Spell Turning would turn the spell back on you, as the description says. Hence, if you target yourself, you are still the target, but you also lose x levels of Spell Turning doing that.
Now, there's the question of the resonance field:
The spell does not, and cannot differentiate between casters (you aren't concentrating on it or anything) and so, as it is written and because the spell doesn't know attacker from non-attacker, there can be a case for the resonance field effect. Definitely--and it could be a very fun, random thing.
However, there is also a good case to be made that since there aren't really two Spell Turnings reacting against one another that this effect wouldn't occur. To this DM, a resonance fields requires this set of events:
Despite RAW, this DM would rule that no resonance field effect would occur (unless the scenario above is met). All of this is to say that once you cast Spell Turning, do not expect to target yourself with spells or else lose Spell Turning levels.
Now, if you cast Spell Turning on yourself while you already have Spell Turning cast on yourself... ;)