It's true that many fruits ripen faster if they're in bags. The reason is that they give off a gas that makes fruits ripen faster. If they're in a bag, the gas is trapped near the fruit. Otherwise, the gas can drift away. It's great that you asked about the color of the bag. It shows that you are questioning what you have heard and thinking about which variables are important and which ones are not. I'm with you here, I doubt the color of the bag matters.

So why should a fruit give off a gas to make it ripen faster? Normally, a fruit would still be on a plant, making all the fruits around it ripen so that they all get ripe at the same time. Can you think of a good reason for this?

I've heard that ripening fruit produces ethylene gas, which helps it ripen.The purpose of the bag is to keep the ethylene around the fruit instead of having it diffuse away. I think the bag needs to be paper, not plastic, because plastic traps moisture - water - that might make the fruit get squishy and moldy. Maybe the advantage of brown paper is to keep out light, which can cause various chemical reactions, or maybe we just say 'brown paper' because paper bags were always brown in the 'old' times.

The reason this happens is that vegetables produce a chemical (which happens to be ethylene) as a signal that causes them to ripen. Ethylene is a gas, so if it is contained somehow, it will overstimulate the fruit and cause it to ripen faster. If the fruit is left out in the open, the gas will just drift away, and the fruit will only get partial effect from it.

I don't know why plastic bags are bad for ripening, though - I'm guessing because plastic interacts with ethylene in some way.

I just heard the answer to your question last night on the Good Eats show of the Food Network. Ethylene gas, which is emitted from damaged plant tissues, causes fruits and vegetable to ripen. Ethylene gas gets trapped in the paper bag thereby exposing the fruits or vegetables within causing them to ripen faster.