IF OBJECT_ID(N'TEMPDB..#COLOR',N'U')IS NOT NULL
  DROP TABLE TEMPDB..#COLOR;

CREATE TABLE #COLOR
(
  ID TINYINT NOT NULL,
  COLOR VARCHAR(10)NOT NULL
)
  
INSERT #COLOR(ID,COLOR)
(
  SELECT 1,'RED'
   UNION ALL
  SELECT 2, 'RED'
    UNION ALL
  SELECT 5,'GREEN'
    UNION ALL
  SELECT 4,'GREEN'      
);

WITH MINCOLOR AS
(
  SELECT MIN(T.ID)MINID,T.COLOR
   FROM #COLOR T
   GROUP BY T.COLOR
)
UPDATE T SET
 T.ID=MC.MINID
FROM #COLOR T
JOIN MINCOLOR MC ON T.COLOR=MC.COLOR

SELECT * FROM #COLOR; 

DROP TABLE #COLOR;