Hi, manon. I've never used UV inhibitor, but it is supposed to help minimize color fading due to sunlight exposure. I don't know how much it will help if the soap is sitting in a sunny window all day, every day, but if it's incidental sun exposure, it should help. I'd experiment with it first. Bramble Berry does sell a UV inhibitor:
http://www.brambleberry.com/Uv-Inhibitor-P3563.aspx. Here's a Soap Queen TV video explaining how to use it:
http://www.youtube.com/watch?v=B-KjcS-L2Sg.
I hope that helps! Hopefully someone who has used the UV inihibitor can offer more insight.