Cement dyes are used to colour concrete surfaces, and there are two main types: water-based dyes and solvent-based dyes. Water-based dyes are made up of fine pigments suspended in water. These dyes are easy to use, are manufactured with low VOC (volatile organic compound) emissions, and are generally considered to be environmentally friendly. However, water-based dyes can be more difficult to achieve consistent colour results across large surfaces. Solvent-based dyes, on the other hand, are made up of fine pigments suspended in a solvent carrier. These dyes are generally more concentrated than water-based dyes, which means that a little goes a long way. However, solvent-based dyes emit higher levels of VOCs and also have flammability concerns. In summary, the main difference between water-based and solvent-based dyes is the type of carrier in which the pigments are suspended. Water-based dyes are easier to use, have low VOC emissions, and are environmentally friendly, while solvent-based dyes are more concentrated and offer long-lasting colour. It's always recommended to consider safety precautions and environmental regulations when choosing a dye for your project list.