Hi Sueellen, It discoloured because it's not UV stable and will degrade with exposure to sunlight. If used externally it needs to be plastered over or painted with an acrylic paint. In relation to the original query I think decorators caulk is the best product for sealing small gaps internally.We used it and found it hard to control (foam shoots out of container) and it looks horrible. When used outside it discoloured quickly.