update on join ?

Hi,

I'd like to update a table  efficiently  where the relevant select-info 
is in another table that is foreign-linked.

Stupid example. 2 tables:
things (thing_id integer, name varchar(100), color varchar(100))
inventory (item_id integer, thing_fk integer references things 
(thing_id), number)

For some reason I'd want to set the number of every red item to 0.
This inventory doesn't contain the color but the foreign key to the 
other table where the color is found.

I tried

UPDATE  things JOIN inventory ON things.thing_id = inventory.thing_fk
SET number = 0
WHERE color = 'red'

PSQL didn't like the JOIN though.
It works like this:

UPDATE inventory
SET number = 0
WHERE  thing_fk IN (SELECT thing_id FROM things WHERE color = 'red')

It's effective but is it efficient, too?

Regards