How to loop in sql?

An alternate method:

MERGE INTO a
USING b
ON (b.aid = a.id)
WHEN MATCHED THEN UPDATE SET a.color = b.type;

You could just do:

UPDATE tablea a
   SET a.color = (SELECT b.type 
                    FROM tableb b
                   WHERE b.aid = a.id)

See this SQL script.

To do that you will have to write a stored procedure using PL/SQL. Here is the oracle page with some info and papers on the topic.

As others pointed out, you can probably solve your problem with a normal DML statement, without any looping involved. But to give you some basics on how to accomplish what you asked for in PL/SQL, here's an example...

DECLARE
    CURSOR c IS 
        SELECT id, aid, type FROM b;
    statement VARCHAR2(200);
BEGIN
    FOR iterator IN c LOOP
        IF iterator.id = iterator.aid THEN
            statement := 'UPDATE a SET color = ' || iterator.type || 'WHERE id = ' || iterator.id;
            EXECUTE IMMEDIATE statement;
        END IF;
    END LOOP;
END;

This anonymous PL/SQL block will iterate through each record in table b, and if b.id = b.aid, it will update table a and set a.color = b.type where a.id = b.id.

This seems to be what you were asking for. It's not exactly an efficient way to go about doing things, since you're firing off one DML statement per row in table b that has b.id=b.aid. But I wanted more to give this as a syntax example. This is just one way to iterate through a cursor by the way; you can also explicitly open cursors and fetch records, but it's easier this way if you don't need to do anything but iterate over the entire result set.