Does Revlon Colorsilk ever expire?

As long as it remains sealed it should be good, Now I wouldn't use a ten year old box, sealed or not. As long as the color hasn't been exposed to air and light it will remain intact. Left in the sun or heat for an extended period of time can cause the peroxide to lose its strength, and cause the color to darken. If kept in a cool dry place without direct sunlight it should be fine for at least 4 years.