Palette texture is a feature for color correction for both old and new versions of the mod. It must be bmp file format 24 bit depth or tga, png files with or without alpha in case of relatively modern versions of the mod. To install, copy enbpalette.bmp to game folder where d3d9.dll of ENBSeries located. Some games may require different paths.
Example of simple palette without color correction and independent from screen brightness here.
Colors from left to right of texture represent pixel colors of screen, for example left black means that original game black will be mapped to black. If it is green, then black pixels of game will be green. Colors from top to bottom represent screen brightness, upper means that overal game screen brightness is black (0) and bottom of the palette texture means that screen is white. So you can adopt colors depending from screen brightness too. When using palette some parameters became inactive, because palette replace them, here they are:
ScreenLevelDay=60 //active only for ColorSaturation
ScreenLevelNight=20 //active only for ColorSaturation
I strongly recommend to make palettes in 3d modelling software, because of quality. May be in future i'll do an editor for palettes, if this new feature will be useful.