Created a new Infopath form.
Placed drop-down-list, in "Drop Down List Box Properties":
- Field name: RGB
- "Entered choices manually" --> "Add...", 3 times
(with Value = "Display name"):
Inserted textbox, in "Text Box Properties:"
Selected the placed buttton --> Add Rule --> When This Button Is Clicked --> Set the field's value
In "Rule Details":
Published - Run - Enjoyed