Is the product usable even if one side is yellow? Will the yellow show in my project?

As long as the product is still mixable (i.e. not cured), it is still viable for use. The mixing process should thin out the discoloration, making viable epoxy resin for your use. Please also keep in mind that when mixed with pigments tints or dyes no yellowing will be noticed. All epoxies with time do yellow. However, when used it will cure just as it did the first time you used it.