The Paint Can is a block added by the OpenBlocks mod. Paint Cans are created in a Paint Mixer with paint of virtually any possible color. A Paint Can can be filled with a (new) color in the Paint Mixer, or a Milk Bucket can be transformed into a Paint Bucket. OpenBlocks defines 16 default colors of Paint Buckets which are available through NEI. These represent the default 16 colors used in Minecraft for dyes. When right-clicking a Paint Bucket with a Paint Brush, the brush will be refilled with 24 uses and the bucket will lose one of its own 24 uses.
The can breaks immediately when left clicked. It will be destroyed if you use anything other than a pickaxe.