on insert rule & primary key

My original post got eaten. Apologies in advance if you receive this
message twice.

I am trying to construct a rule that performs an UPDATE if specific
conditions are met in an INSERT statement. Limiting UPDATE's SET
action to just the new row by testing for the new primary key is
failing for some reason. Yet if I eliminate the test, all rows in the
table are updated.

The actual rule I'm building must handle several OR clauses in its
conditional test, so I've included that in the following sample. The
output I would've expected would have both the Carlos and Miranda
inserts yielding their favorite color, azul.

Any suggestions on how I can construct the rule to automatically and
correctly fill the fav_color field?

Thanks in advance!
Scott

CREATE TABLE colors (
clrs_pkey SERIAL PRIMARY KEY,
first_name text UNIQUE DEFAULT NULL,
fav_color text DEFAULT NULL
);

CREATE RULE color_rule AS ON INSERT
TO ONLY colors
WHERE
first_name = 'carlos' OR
first_name = 'miranda'
DO UPDATE ONLY colors SET fav_color = 'azul'
WHERE clrs_pkey = NEW.clrs_pkey;

INSERT INTO colors (first_name, fav_color) VALUES ('carmen', 'verde');
INSERT INTO colors (first_name) VALUES ('carlos');
INSERT INTO colors (first_name, fav_color) VALUES ('rocio', 'rojo');
INSERT INTO colors (first_name, fav_color) VALUES ('miranda', 'negro');

test=> SELECT * FROM ONLY colors;
clrs_pkey | first_name | fav_color
-----------+------------+-----------
1 | carmen | verde
2 | carlos |
5 | rocio | rojo
6 | miranda | negro
(4 rows)