CORIO2 scalers use YUV (YPbPr) signals for internal mixing and image generation.
If you want to convert an RGB colour value to the respective YUV values for keying and background colours, then use the following Excel spreadsheet:
Note that there are 2 color-spaces that can be converted to, depending on your output resolution (NOT your input resolution).
See also YCrCb values for various colors if you cannot open this file.