Only some RGB values are supported on some units (partial implementation on all updates in the past few years except for the non-TI Elite units). The RGB value is limited to 8-bits per channel for Red, Green, Blue, and Alpha.
It's recommended that you provide an index color in addition to the RGB color. If an RGB color is supported, it will be preferred over the indexed color...otherwise, the indexed color is used. If an indexed color is not provided and the RGB value is not supported, it will be transparent.