Incorrect white balance

[GungGung]

I took this picture and many others like this with

Cannon EOS XTI
Apertur Priority (Av) - F3.5
White balance set to Flash
ISO 400

lens - Tamron 18-270mm DiII

Flash - External 430EX set at ETTL mode (both the flash and the camera was set in 2nd curtain sync (by mistake, left over from last shoot)

All the pictues turn out to have these deep red color. Most of the time, the camera sets the speed to 1/30 or more.

Can any one tell me what cause this color shift? Thanks

[Elmo]

At f:3.5 the existing lighting furnished the majority of the lighting (notice the background is equally bright). The flash only functioned as a fill. The existing lighting is red compared to the flash WB setting.

[Gonzo13]

Elmo makes sense. If you shot it in RAW then you can change the WB to a tungsten or maybe a white fluorescent. If not you might be able to increase the blue in photoshop. Oh yeah and pictures are supposed to be 800 pixel max on the longest side.

[Doug Pardee]

The mistake was using Flash WB along with Av mode.

If you intended a simple flash photo, you shouldn't use Av mode. That's your "fill flash" mode on Canon DSLRs. For simple flash photos, "P" mode works okay. Learning to use M mode for flash photography is worthwhile in the long run.

If you wanted the fill-flash Av mode, then you shouldn't have used Flash WB. The lighting is basically from the incandescents. A Tungsten WB would have been closer, especially if you used some WB Shift toward the blue (Tungsten WB is calibrated for 3200K photofloods, not for 2600K home lighting). Technically, for fill flash you should also have used a CTO gel on the flash to make its light be closer in color to the ambient.

[GungGung]

Thanks everyone for the pointers. I learn something today!