This is actuatually not a bug, but a inconsistance with the FX documentation. On the wiki page ( <a href="https://wiki.roll20.net/Custom_FX" rel="nofollow">https://wiki.roll20.net/Custom_FX</a>), it is said that there are the attributes "startColor" and "endColor" to define the FX colors. However, these names are wrong. The correct names are "startColo u r" and "endColo u r". I've spend some time trying to create an effect when I finally realize it.