Urethane resins will yellow with age. The question is how much and how long?
In early October of this year I placed a piece of Smooth-Cast 300 resin outside on the patio table. This area is mostly in the shade except for some sun during the morning and evening. Another piece of SC 300 was placed on my desk where filtered sun hit for no more than an hour a day. The room wasn't very well light and the sun light was brief.
Today I compared October's resin with the Smooth-Cast 305 resin I just cast today.
Above you can see the two on the left was made in early October and the one on the right was just made less than an hour ago. The top left was the resin left outside and the bottom left was the resin left on my desk indoors. You can see a noticeable yellowing of the one left on the patio for under 3 months. Although it isn't as yellow as the picture my screen shows (yours may differ), in person it is definitely a noticeable change. The resin piece that was indoors I can't see the difference between it and the new resin piece. So that is good at least. Keeping in mind that the October resins were 300 and the new one is 305, but it shouldn't matter too much.
It will be interesting to see how the changes go now. I will leave a new 305 outside on the patio and place another indoors. This will help see if there are additional changes over time.
One of the downsides to resin is that it will yellow with time, but how you protect/keep your resin sculpts determines how quickly this yellowing occurs and at what intensity.