Why should we use only white cloth for wet pack?

Because -

In order to colour the cloth, a combination of different dyes are used that are made using different chemicals. The toxins in these dyes can be very harmful to the skin, especially when the cloth is wet and kept on the body for a long time. It is, therefore, best to use a white cloth that contains no artificial dyes.

Additionally, we see toxins being absorbed by the wet pack. Many times the colour of the cloth becomes yellow or starts to smell after use. This indicates that toxins are being released from the body. This helps in keeping track of the detoxification process which would be impossible to do if we use a coloured cloth.