Shooting a restaurant assignment at Harbor Lights, I found that any shot showing their signature view also had really long-scale scene. So even when blowing out the highlights outside, the foreground subjects were too underexposed:

So I needed to add light to the room, and bouncing a strobe off the ceiling is almost always the practical way to do that. But the ceiling at Harbor Lights was red. REALLY red. The bounced light was way too warm:

If I could put a colored filter over my strobe to skew the color away from red (i.e., toward blue), I might get a usable result. For such purposes I carry a collection of colored gels, ready-to-go with wings of gaffers tape. They add almost no weight to the camera bag and are very compact.

Knowing that my "cooling" gel wasn't strong enough to balance the strongly red ceiling, I tried the blue filter:

Which was obviously too strong, but when bouncing, one can moderate the color by partially uncovering the flash head like so:

The resulting color is now a little too purple:

I suppose if I were really careful and wanted to reduce the color-correction that I'd do later in PhotoShop, I might have added some of the green gel, too.

But the result was perfectly useable:

And a whole lot better than no added light at all: