One of your main tricks early on is going to be using spells like silent image to trick you enemies.
How that works: you can create an image of anything you like (within size limits). As long as you concentrate (a standard action) you maintain this image. If an enemy interacts with the image (such as an attack) it gets a saving throw to disbelieve it. This is great to send a couple of enemies off on a tangent while breaking up the rest of the group. The main limit to this spell is your imagination.
- Gauss
Try to take a few other spells that you can mix with your illusions, for example, if you make an illusion of a demon summon real ones.
The best advice I can give you is to spend 30 minutes with your GM going over expectations for how the GM will handle illusions. Get a clear understanding of what the GM expects in your descriptions of the illusions you cast, and try your best to get a clear agreement between you and the GM on how specific illusion spells will be dealt with.
I used to play a 2e Illusionist. In fact he was my most powerful and highest level character until my 4e ranger passed him up. Early on, before I had my own heart-to-heart with my GM, using illusion spells was a very frustrating experience. What I thought was happening and what my GM thought was happening were not always the same thing.
For example, the first time I tried to cast an illusion of a dragon, he decided that since I had never seen a dragon before that my dragon was less believable than I thought it should be.
I learned that the best way to have my illusions work was to have real effects and illusions mixed up so that when an illusion was cast, the NPCs already had seen something real that made the illusion more believable.
I loved playing an illusionist, but I had a good GM who was willing to work with me. I've heard other people complain about how their illusions were never any use because the GM meta-gamed around them.
So again, make sure expectations are clear.
Color Spray is certainly a must at low levels, and even once you get into high levels, it can be a good last resort spell. A bit ago I had an 8th level sorcerer use it in a tight spot.
Make sure you also have a variety of other spells, as otherwise there'll eventually be a time when you're fighting an enemy with high will saves, and without those other spells, you could end up stuck.
As others have said, having both conjurations and illusions can be pretty fun and confusing to the enemies, but depending on your GM, they might meta game around it.
Have you given any thought to what bloodline to take? Serpentine has a bloodline arcana that's useful - it lets you treat animals, magical beasts, and magical humanoids as if they were humanoid for mind affecting spells.
If you were thinking at all about multi classing, taking a level of oracle could have interesting results. The one drawback is that your spell progression seems very slow, but there's a revelation of the heavens mystery (Awesome Display) that lets you treat illusion [pattern] spells with HD factors as if the target's HD is equal to their normal HD minus your charisma modifier.
if you make your (silent) images fun, the GM is less likely to disallow them.
You can try to avoid any combat by just frightening your ennemies or ather tricks because straight up "I deal damage or suck and save" is not really the illusionist way. Your group should be asked first how to handle stuff.
Here's a thought, since your doing illusions. If you take 1 level in Oracle and take the Heavens Mystery and choose the Awesome Display revelation, all your illusion(pattern) spells is treated as if its total number of Hit Dice were equal to its number of Hit Dice minus your Charisma modifier.
So if you use something like hypnotic pattern, a level 2 sorcerer spell, if you roll two 1's on the 2 d4 you have to roll to determine the HD of creatures affected, with this you basically add 3 to it, if your Charisma is at least a 16. You add more if it's higher of course, which I'm sure it is since Sorcerers rely on Charisma for their spells
This works with color spray too. Instead of creatures not really being affected by the spell if they are 5 HD or higher, with this, they get knocked unconscious as if 2 HD.
One other thing, taking 1 level in oracle also gives you access to cure spells.