Query to return modified results at runtime?

I have a query:

SELECT * FROM testtable;

Where the results are:

ID Color
--- -------
1 Blue
2 Red
3 Green
4 Orange

How would I rewrite the query to return results where the colors are
replaced by letters to give the following results?

ID Color
--- -------
1 A
2 D
3 B
4 C