Not all white paper products, like coffee filters, are actually white. Many have a slight blue tint because people tend to perceive that as more pleasant than pure white.
It is better to use a quality gray card, because accurate gray tones have equal amounts of red, green, and blue.
As far as white balance tools that fit on the lens, many lenses have different front end diameters so you might wind up with an entire collection of BRNO White Balance Lens Caps.
I recommend this small group of tutorials -
Tutorials on Color Management & Printing
Understanding White Balance
Digital images have 3 color channels - Red, Green, and Blue (RGB). RGB is an additive color model -
RGB color model - Wikipedia, the free encyclopedia
255 is the maximum of each color, so pure white is 255 in each color channel and pure black is 0 in each color channel.
Pure red would be red = 255, green = 0, blue = 0. Pure green would be 0, 255, 0 and pure blue would be 0, 0, 255.
This is white red = 255, green = 255, blue = 255
This is red = 250, green = 250, blue = 255
This is 18% neutral gray (red = 209, green = 209, blue = 209)
Note: Like pure white/black, all 3 color channels have the same value and no 1 color dominates.
