To be honest, cobalt is a transition element and potassium burns with a pale violet flame, so it might be the fact that you may see a reflection which is of barbie pink color. Anyways, Warren windows use similar techniques to color their windows and bring tint into it.

oh, we just did an experiment just like that. All I remember though is I think it had something to do with spectral lines and the cobalt blocking out some of them, allowing you to see the potassium flame. sorry I can't fully remember

Hmmm. That's kind of strange because potassium puts out a VERY intense violet color when put in a flame. I've never even heard of any special glasses being needed.

yeah, that's wierd.

Moreover, why are you burning oranges...?

mmm... oranges....
ok, then

maybe i'm thinking of calcium or sodium... some of those produce orange flames... when we did the experiment, i don't remember any the color violet as you described, but maybe i just don't remember or performed the experiment wrong.
let me go check this on google...
well, it says it produces a lilac flame. tee hee.
anyways, the real question was about what makes cobalt glass change the color of a flame, wasn't it? oh well, i shouldn't argue about what i don't know... P

sodium burns orange

I suspect the reason is that cobalt glass is absorbing some wavelength that carry certain color, for example blue wave, but i couldnt remember which wave exactly cobalt absorb.

In contrast to normal glass which do not have any metal impurities, they do not have free electron to absorb these wave energy