Type I is pale red
Type II is clear or "straw colored" (rarely used)
Type IV is green
As far as I know, there is no Type III in use for civil aviation. I've never seen it mentioned anywhere.
The color is important to help the deicing crew tell where it's been sprayed, and to identify the type, since your time of protection, or "holdover time", varies with the type of fluid and type of precipitation.