I added a Sepia Tone effect with 4 different modes: Tint, Matrix, Duotone and YIQ. The download link is in the first post of this thread.

- Tint: the pixel is converted to grayscale and an offset is added for each R-, G- and B-channel
- Matrix: the pixel's RGB color is multiplied with a matrix which returns a sepia tone
- Duotone: the grayscale value of the pixel is used to blend beween two sepia'ish colors
- YIQ: the pixel's color is converted from RGB color space to YIQ color space, modified to a sepia color and translated back into RGB color space

Sepia effect:


When adjusting the alpha parameter, you can blend the Sepia tone with the original image to control the style of your game. Especially the YIQ- and Duotone-mode are useful. If you change the YIQ-target color and tune down the alpha, you can make the scene "feel" colder or warmer. With the Duotone-mode you can produce arbitrary duotone images.