Update: vignetting found at wider than 24mm
One weakness of many DSLRs is the poor performance of auto white balance in artificial light compared to point-and-shoot cameras. Check out dpreview if you don't believe me. To get perfect white balance, there are after-market products such as the ExpoDisc (a little under $100) and the SpyderCube ($59).
For those with different budget priorities ^_-, there are cheaper solutions such as white balance lens caps. I got the Mennon brand one for 72mm. The plastic build quality is proportionate to the price. (For more discerning buyers, there's the Brno white balance lens cap.) The middle part of these lens caps are made of a somewhat translucent white plastic material. To use them, you put them over your lens then take a manual white balance measurement. As lens caps, they are also economical substitutes for the original lens cap (so you can avoid losing them), and they include a convenient strap.
When buying a white balance lens cap, check to make sure that it includes the adapter for your filter size. The adapter is screwed on to the lens like a screw-in filter and the lens cap then clips on to the adapter.
I tested the white balance with and without the lens cap. Here are the results:
Auto white balance:
Incandescent white balance:
Manual white balance without lens cap:
Manual white balance with lens cap:
To me, the white balance lens cap seems a little on the bluish side of white. I wouldn't rely on these lens caps for white balance unless the manual white balance is not working for some reason. For flexibility of white balance adjustment I still take shots in raw, then click on white or gray parts of the image such as the subject's eyeballs to change the white balance.
One of the issues of using it as a lens cap is that it may not fit with the lens hood. The lenses I've tried it with (Tamron 28-75, Tamron 17-50) have petal type hoods which could be used with the lens cap if the eyelet for the lens cap strap is turned toward the corner between the petals. My lens hood also has grooves which allow the lens cap eyelet to 'clip' on to the lens hood with friction. For barrel type lens hoods with no grooves, the eyelet for the strap may make it difficult to fit onto the lens with the lens hood attached.
Another issue with these lens caps is the possibility of vignetting. On the Tamron 28-75, I saw no vignetting. I could leave the adapter on all the time.
No vignetting at 28mm (Tamron 28-75, 67mm filter size)
However, with the Tamron 17-50 VC, the edges of the adapter were visible when I was wider than 26mm.
Vignetting from the adapter at 17mm
Vignetting from the adapter at 24mm
Vignetting inherent in the lens (not the adapter) at 26mm (on Tamron 17-50 VC, f/2.8)
Vignetting inherent in the lens (not the adapter) at 32mm (on Tamron 17-50 VC, f/2.8)
This makes the adapter pretty much unusable for my lens. What I do therefore is to keep the white balance cap strapped on to the lens. When I'm using the camera, I attach the lens hood then "clip" the cap using the grooves of the lens hood. When I'm putting the camera away in my bag, I remove the lens hood and white balance lens cap, and I attach the original lens cap.
Conclusion: for white balance, these are a little bluish. I would still use raw. As lens caps, they may be ok to avoid losing your original lens cap, but check whether your lens hood is a petal type or the barrel type. If it's the petal type it's probably usable. If it's the barrel type, you probably can't use it. In any case, vignetting is a possible issue for lenses wider than 26mm.