I'm interested in building an Illusionist that works well from level 1-15 ish. I've never tried this in pathfinder and could do with a hand with sifting through the options :)
Build parameters: Ideally PFS legal, but this is for an AP (maybe Iron Gods) in a home game. Bonus points if it can be done with a single class and non-gnome (Not a hard requirement, I'm still interested if gnome is simply way better than the other races).
Should I initially focus on color spray? What after that? Is Shadow Conjunction worth building for?
The single class part is harder than the non-gnome thing. Gnomes are only giving you 1 to the DC, but there are a few classes that could contribute.
-1 level of Heavens oracle for the Awesome Display revelation is fantastic. You could stick with oracle but they don't get all that many illusion spells.
-Sorcerer, wizard, or Arcanist to really stack the illusion spells on your spell list, and all of the [pattern] spells you cast with that class will be affected by the Awesome Display
-At level 6 you could go into the Veiled Illusionist PrC. This is better if you went straight oracle because it gives you more illusion spells and some cool other toys.
-if you're not going gnome, go human to get Spell Focus (illusion) and Combat Casting at level 1. If you're going to be using color spray a lot, you'll need Combat Casting early and can then retrain out of it later on.
Gnome has more to offer than +1 DC. They also get access to Effortless Trickery, which is a simply amazing feat. Being able to maintain an illusion as a swift action then use your other actions to cast and maintain other spells is really strong. I can't count the number of times I've, say, maintained an image of an illusory threat, then stepped in and cast color spray. Or had two image spells going on simultaneously while still being able to shoot magic missiles.
Gnomes also get Threatening Illusion, which can be great for summoning up illusory threats on the battlefield.
Gnomes also get to make threatening illusions which give your allies flank.
I've done well with a human wizard, and using colour spray, patterns, and the core x image spells. You need a GM who will work with you to some extent like the notion that a standard action is required to interact with an illusion.
There is feat that let's you use slight of hand to disguise that you have even cast a spell which helps.
I've found shadow evocations less useful then just having evocations.
I'm a sucker for the Bard. Spellsong + Effortless Trickery allows the maintenance of three separate illusions. Plus high Bluff and Diplomacy seem essential for any trickster.
I like bards too but they don't really get enough spells per day which is a shame since maintaining three illusions plus a bardic performance sounds awesome.
I'm leaning towards wizard as being the most practical class for this mostly due to flexibility.
Illusions are best used as a support ability and depend a lot on your DM point of view on the way Illusions work, like if your dm gives a will save to npc as soon as they see the illusion, might not be useful...mostly why, it's often better to play a Bard and gnomes bards are frankly excellent.
Given the flexibility of illusions, I'm a fan of just going for Gnome Sorcerer, with either the Arcane or Fey bloodlines. You'll have wiggle room to be able to pick up some buffs, debuffs, or direct damage spells for times when the illusions might not be the best option.
You could also run with an Arcanist, and make the most of the ability to spend reservoir points to boost spell DCs.