Colour of urine

Question
I've noticed that there are some orange wet marks on the sawdust in my female guinea pigs hutch. At first I thought it was blood in the urine, so I placed her on some paper towels to see what colour her urine is.
Her urine comes out clear and completely colourless, but after 20 to 30 minutes it changes to a light orange colour. I've searched the web and seem to recall seeing a page that said it is caused by something in the urine reacting with the air, but i cannot find the page again. Have you any ideas?
Thanks

Answer
Often the acidic particles in the urine can interact with the bedding, causing it to change color, I use pine shavings, and my bedding also changes into an orange/red color after it is urinated on, even though the pig's urine is clear. Most likely the ammonia deposits in the urine cause this.
Brittany