The Paint Mixer is a block from the OpenBlocks mod. This block is used to turn Milk Buckets, Ink Sacs, Yellow Dye, Magenta Dye, and Cyan Dye into an assortment of different colored Paint Cans. Any color can be created in the Paint Mixer GUI by either entering in the color hex code value (e.g: #FFFFFF would create a solid white), or choosing it in the color palette.
Put a Milk Bucket in the top slot, then put some Black, Yellow, Magenta, and Cyan Dyes in the according slots, choose a color, and click the Mix button. After the process is complete, the Milk Bucket will be turned into a Paint Can of that color.