According to many online reviews, Redken hair color charts are reliable, as of 2015. There are several charts, each corresponding to a specific Redken product. The charts are not interchangable.
Redken has several hair color products, including Shades and Color Gels. Ranging from permanent to demi-permanent, it's important to make sure that the color chart corresponds to the product being used. Each product works differently when applied to the hair.
Some Redken hair color, such as Shades EQ, is designed to cover gray and refresh color. On the other hand, permanent hair color is designed to interact with natural pigment to create a final color. Redken is a salon brand, which means it's always best to consult with a professional when choosing and applying hair color.