As with many urethane rubber and resin formulations, color variation from batch to batch is common and is a function of color variability of raw materials that manufacturers like Smooth-On receives from suppliers.
Color of these types of products can change from batch to batch made in our factory and can vary from light to dark.
Whether light or dark, color has no effect on functionality or performance of the cured material. Physical and performance properties will be the same from batch to batch and you can use the material with confidence.
Urethane rubbers and plastics are not UV resistant and cured material will darken in proportion to exposure (intensity and duration) to ultra violet light.
The exception is when a particular urethane rubber or plastic is made with UV inhibitors to delay the effects of UV light.