The rear walls are covered in a cloth material (that’s why it has the diagonal lines in it), while the floor is metal. The colour of the cloth isn’t the same on all UH-1s, though:
Interior of a Bell UH-1H Iroquois (Huey) Helicopter used extensively in the Vietnam War by Mary Harrsch, on Flickr
Bell UH-1D (65-10077) Cabin by Daniel Berek, on Flickr
Bell UH-1H Iroquois (68-16623) Main Cabin by Daniel Berek, on Flickr
Interior of a Bell UH-1H Iroquois (Huey) Helicopter used extensively in the Vietnam War by Mary Harrsch, on Flickr
Bell UH-1D (65-10077) Cabin by Daniel Berek, on Flickr
Bell UH-1H Iroquois (68-16623) Main Cabin by Daniel Berek, on Flickr

That said, dirt on the floor is simple enough: take a somewhat large brush (say, about 3 or 4 mm diameter), dip it into some earth-coloured paint, and then wipe of the paint on a piece of cloth or paper from a kitchen roll. Then lightly go over the floor of the model with this, so that the paint will catch on the higher parts of the floor without actually covering well. This is known as drybrushing because you’re painting with a brush that is (almost) dry. It takes a little practice, but it’s a very useful technique to master.
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