How is Vf handled when mixing LEDs?

I'm usually running LEDs with similiar Vf in series, or using a current regulated driver which renders the question rather moot.

However, I'm curious how LEDs handle electric current when more varying LEDs are used in series on a fixed voltage source.

For instance, if blue and red LEDs are run in series, and a constant voltage source is applied, what's the characteristic Vf change if the voltage source is greater than the total Vf of the LEDs? Does the Vf difference matter between the different LEDs, or is it still just a matter of the total Vf of the series?

Right now I'm building a lot of stuff using laptop bricks because I just happen to have a lot of laptop bricks. Mixing red and blues, and just using the total Vf to calculate my circuit seems to be working just fine, but I want to make sure this is indeed a correct assumption rather than just luck.

There is really good information on the LED Vf effects in the Lumileds data sheets, extensive curves, etc. The general nature of these curves applies to most brands.

You can get away with a lot running a mixture in series.

In general, red is more sensitive to temperature changes than blue, both for Vf and output. Brand name blue LEDs can usuallly take quite an overdrive beating, reds - not so much. The exception to this is the Lux III red and red / orange - quite a tank.

It probably does not matter, but just an FYI - for electrically active thermal pad packages, the red and blue have different polarity pads.