Given a named CSS color string, generate a random hexadecimal (hex) color code that is dominant in the given color.
- The function should handle
"red", "green", or "blue" as an input argument.
- If the input is not one of those, the function should return
"Invalid color".
- The function should return a random six-character hex color code where the input color value is greater than any of the others.
- Example of valid outputs for a given input:
| Input | Output |
|---|
"red" | "FF0000" |
"green" | "00FF00" |
"blue" | "0000FF" |