The Paint Bucket is an item used as a multi-block Metadata-altering tool in Debug Mode. (The Paint Brush is used for single-block Metadata-altering). For some blocks it will add a color tint. For others, it will cycle through the other block textures of that type.
Paint Bucket Hotbar Icon
To change a block’s Metadata with the Paint Bucket, you must first use the Cursor to select a target area which contains any of the listed blocks. Then, while holding the Paint Bucket, right-click any block within the target area, and any blocks contained in that area will be altered into their next Metadata version.