It depends on the combination with or without an article. Here is an overview:
Nouns that end in -ung are always feminine. The word is not plural but singular because it is an uncountable noun like "clothing".
The subject is wir and the thing that is used is the clothing. Therefore "Kleidung" is in accusative. There is a definite article "die" so you have to look at Weak declension. So it is Weak declension, accusative and feminine, therefore the adjective should be "grüne".
I gave the table as I think it is useful to think about different situations but I should have explained that the noun is not plural.
Does this sentence mean anything in German, i.e. is there a context to say this?
Maybe the context is someone explaining a hospital's policies for what is worn in the operating theatre, as opposed to blue scrubs/clothes?
There are arguments about green, as it is opposite red on the colour wheel, being more suitable for surgical operating clothes, than blue.
July 13, 2017 - It seems to me that benutzen would even be the better choice for this sentence, because the object (die grüne Kleidung) is a material object. I think you would be understood, regardless. https://answers.yahoo.com/question/index?qid=20110329050133AAMQnT0
Sometimes I just don't hear her well. And I don't like to depend on the clues. What I heard was "Benutzen die bunde kleidung." Which would sort of mean "use the colorful clothing," and that sounded reasonable. I think she speaks too fast. That's, of course, avoidable by choosing the slower option, but I'm trying to train my ear to hear. It's not working out so well....LOL
... To build a fire.
... To mop the floor.
... To block the draught.
And whatever strikes your fancy.