I did do this once for a friend in a rental home (the landlord was fine w/us re-painting a small bathroom the previous tenants had painted NEON blue!) I would not recommend mixing semi-gloss and flat though. I mixed a beige with white, but they were the same manufacturer and the same sheen (I think satin). It came out fine, and is holding up well.
But, I am sure the experts here need to chime in with this...I can only tell you my one experience with it!

In my last house I had many quart samples of gold paint that I had used as testers. When it came time to sell, we still had one bedroom that we pretty much used as a junk room and had never painted. I told DH to throw them all together and see what he came up with. The result was the prettiest color in the whole house.

This is what I found online:
---------
Create a list of all paint you have stored. If the paint ... is latex, and still looks fresh, try mixing it with other odds and ends of latex paint. The color will turn out beige, gray, or muddy. If you like the resulting color, use it in your house. Otherwise, itÂs fine for a garage, basement, or as an undercoat. As a rule of thumb, do not mix exterior paints with interior paints and remember oil and water do not mix, do not blend oil-based paint with latex- or water-based paint.
-----------
Any experience with doing this or tips?

The above post captures it well.

* Interior & Exterior paints will mix just fine (as long as it's "Latex w/ Latex").
* Many dark tint-bases are Int./Ext. combo-bases.
* Flats & Semi-glosses will mingle just fine too. Flats just have more pigment to them. Depending on proportion, you'll just end up with a "Satin-ish" sheen.
* You'll probably have Parma's result...the prettiest color!

Faron

I spent a lot of time trying to find just the right red for a feature wall a couple of years ago. The first gallon I bought was too red. The second gallon was too purple. The third gallon was too brown. Having nothing to lose, I mixed all three in one 5-gallon bucket. It was PERFECT!! Again - the best color in the house!

What a great post!

As you've seen with the responses...you can definitely mix paints so long as they are the same base (water or oil).

It really brought back a fond memory for me.
When I was about 14 I remember coming back to "the shop" and seeing all the guys grabbing every gallon off every shelf and pouring them into a big mixing vat. Every left over gallon from years back to the last week. Any questions were met with Dad just saying "dump it in there!". Everything from whites to neon greens. Mixed it all up and it came out a lovely beige.

He took a color sample on a card and sold it to a hotel job he just landed. We did an entire 5 story building with that stuff...and it looked like the day it was painted for 16 years, until Fairfield bought the place and renovated everything.

For 16 years I drove past that place and smiled...thinking of Dad...as long as I could forget hanging off that thing on a boatswain's chair actually getting the stuff on!

Mix 'em as you see fit!

Here is the bottom line from someone with a lot of experience is combining different colors for charity paint. If this is a home that you live in and will continue to live in, do not mix colors. When you need more paint for touch up or you like the color and you want more, it can never be duplicated to the exact sheen and color. Mixed paint also tends to be dull and muddy. Vibrant, clean colors that look good on a wall need the right base with the right tints. Mix your latexes and oils for sheds and barns. Your home deserves better.

Here is the bottom line from someone with a lot of experience is combining different colors for charity paint.

I assume you mix (or mixed) using only the colors you had on hand with the intent of ending up with colors that were pretty, complex, and germane to recent trends. Sounds interesting, corey. You had my full attention at "clean colors", btw. :-)