颜色取反

方法即是

0xFFFFFF - $oldcolor

 

转自

http://zhidao.baidu.com/question/143793592.html?push=ql

 

echo dechex(0xFFFFFF - $oldcolor);

把原文贴出来吧，不然可能不知道怎么来的。

You can invert a color by subtracting each of its red, green, and blue components from 255. In other words:

    new_red   = 255 - old_red

    new_green = 255 - old_green

    new_blue  = 255 - old_blue

You can break a color into its components, use this technique to invert the components, and then use RGB to recombine them into the inverted color. However, there is a much simpler method.

If a color's components are given as R, G, and B, then the color is represented as:

    R + 256 * G + 256 * 256 * B

Now consider the representation of inverse of this color with components 255 - R, 255 - G, and 255 - B:

    (255 - R) + 256 * (255 - G) + 256 * 256 * (255 - B)

  = (255 + 256 * 255 + 256 * 256 * 255) -

    (R + 256 * G + 256 * 256 * B)

  = &HFFFFFF - the_original_color

So a simpler method for calculating the inverse of a color is:

    new_color = &HFFFFFF - old_color