Thanks for reporting it.
We will support rgb value for the legend in the future.
Currently there is a workaround.
You can add a color list to the file Origin.ini in the User Files folder.
[Color List]
Black=0 0 0
Red=255 0 0
Green=0 255 0
Blue=0 0 255
Cyan=0 255 255
Magenta=255 0 255
Yellow=255 255 0
Dark Yellow=128 128 0
Navy=0 0 128
Purple=128 0 128
Wine=128 0 0
Olive=0 128 0
Dark Cyan=0 128 128
Royal=0 0 160
Orange=255 128 0
Violet=128 0 255
Pink=255 0 128
White=255 255 255
Light Gray=192 192 192
Gray=128 128 128
LT Yellow=255 255 128
LT Cyan=128 255 255
LT Magenta=255 128 255
Dark Gray=64 64 64
Custom1=128 255 128
[Color List]
The last row is the custom color. Then you can use the custom index 25 in the legend.
Sam
OriginLab Technical Services