All clear cables turn green after awhile, it's a sad thing as i find Clear cables a lot softer, and matches any kind of IEM compared to Black.
If the custom monitor is made of Hard acrylic, then no there will not be a change in colour. However, if you have a part soft vinyl or part/full soft silicone custom monitors for example Westone uses vinyl tips, UM's Aero SH uses Silicone tips, and Sensaphonics along with ACS use plain Silicone. Then the soft part, being the Vinyl and the Silicone will turn yellow overtime. You are able to slow down the change in colour by keeping your monitors clean with alcohol-free audio equipment wipes, and a microfiber/eyeglasses cloth.