Currently exported DOCX document may be visualized by MS Word with different highlight colors. There is a limitation in the document format which RadRichTextBox doesn't respect, and this could be avoided by unifying the palette in RichTextBox's UI with the one in MS Word.

In RadRichTextBox, the highlight color picker has more colors than the one in MS Word. That is why when exporting paragraph with one of the additional colors, MS Word (the DOCX format) won't use it but will fallback to one of the available colors.