Dyeing Fabric Using Cold Water?

I recently purchased 100% cotton bathroom rugs that were made in India. They must be washed in cold water and line dried to prevent shrinking. While I really like the rugs I do not like the color. The name of the color illustrated in the catalog was sage, but they are more of a lime green.

I would like to dye them a darker color of green. According to Rit, you cannot use cold water to dye. Does anyone have any ideas on a way to dye these rugs using cold water? Thanks for your help.

By it's.only.me from NE PA