This filter will usually be found mounted inside the ST-9E's filter wheel. As shown in the photo, it appears magenta in reflected light.
RGB CCD filters are designed for artistic astrophotography. They can, in theory, be used for photometry or other scientific measurements as well, but they do not correspond (even approximately) to any standard filter. There is no such thing as standard RGB filters, either; each manufacturer has its own philosophy about the properties an ideal RGB set should have.