Urea is a chemical compound that can make your tie-dye brighter. It comes in a white, powdered form and can be found where you buy your fabric dyes. Add about a quarter of a cup per quart of dyeing liquid. This helps the dye penetrate the fabric and produce brighter, more vibrant colors. Urea is a humectant, so it works by keeping the fabric wetter for longer periods of time, thus allowing the dye to penetrate deeper.