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