节选自Duke在Coursera上的公开课：程序设计与Web入门。

How to make a filter in any color

The values in the table were determined by plotting a piecewise linear function for each of the R, G, and B values vs. the average value. To keep black pixels black and white pixels white, the desired functions are pinned at (0,0) and (255,255). The third point (where the function changes slope) is determined by plotting the R, G, or B value of the desired color vs. 127.5.

Consider any color any color with R value = Rc, G value = Gc, and B value = Bc. The filtered pixel has R value:

R = Rc/127.5*avg                      for avg < 128
R = (2 - Rc/127.5)*avg + 2*Rc - 255   for avg >=128

Similarly for G and B, where you would substitute Gc or Bc for Rc in the above formula.

To apply this formula and create a colored filter of your choice, use a color picker tool to determine the RGB content of any color you would like to use, such as teal (17,170,153).

Since for teal, Rc = 17, Gc = 170, Bc = 153, so

R = 17/127.5*avg                          = 0.13*avg        for avg < 128
R = (2 - 17/127.5)*avg + 2*17 - 255	  = 1.87*avg - 221  for avg >=128

G = 170/127.5*avg                       = 1.33*avg        for avg < 128
G = (2 - 170/127.5)*avg + 2*170 - 255	= 0.67*avg + 85   for avg >=128

B = 153/127.5*avg                       = 1.2*avg         for avg < 128
B = (2 - 153/127.5)*avg + 2*153 - 255	= 0.8*avg + 51    for avg >=128

Now you can use this formula to make a filter in any color!