Hmmmmm, the only 'red maples' I know of are the northern hardy diciduous Japanese Maple. They don't make good Bonsai because the leaf size can't be reduced at all. Regardless, all Maple are not indoor but hardy diciduous trees, Bonsai or not. I'd go back to where you bought it and ask about what variety it is.

I went a talked to people at the place where I bought it and they told me that I should keeep it inside since it wouldn't survive the outside weather.
Am I not giving it enough sun?
Why are the leaves turning green?

Where do you live?

Plants need light, so I'd say, yes, it needs more light.

If it's a maple, it belongs outside.

Josh

He/she lives somewhere in Canada Josh. Still doesn't have an idea of what kind of Maple it really is so it sounds like a Mall-tree?
Since 90% of them are northern hardy I'd guess the people selling it are clueless about the variety and it's probably 100% greenhouse grown?? so they have no idea...a leaf pic would be nice to be able to ID it?

With the leaf color that's being described I'll take a Trident variety for $200!! Also, the Full Moon break bud as a redish color then turn green as they mature.

Bob

There is Japanese red maple (Acer palmatum) and there is Red maple (Acer rubrum). Here in Kansas the Japanese and the regular Red maples are just fine, I do not know what there limits are. Many JRM can be used for bonsai while some do not reduce as well. I believe it was a member on here that said they had a Red maple tree. Not sure where they were, I believe the idea was that it didn't reduce greatly but it was a workable specimen.

Look up both and see if they would even live outside where you are. If so they would need to be there. Deciduous trees have to have a dormant period.

It's a Japanese Red Maple and I live in Calgary, Alberta.
Ever since the leaves stated turning colours I moved it to a sunnier place and the leave are still steadily turning green/yellow.
When I went to buy my bonsai, it was in the indoor plant section. All the bonsais were. Which is why IÂm worried about putting it outside, even though weÂre having a wave of nice weather.

(Full)
http://img132.imageshack.us/img132/9448/redbonsai.jpg
(Leaves)
http://img132.imageshack.us/img132/2936/redbonsai2.jpg

That definitely appears to be a Japanese red maple. Some quick searching shows that JRM is hardy to USDA zone 5a while you are in approximately 3b. So I guess putting it outside for the winter is not really an option. However, that tree will not live for more than a year or two as a houseplant, it has to drop it's leaves and take a rest at some point. This would mean moving it to a cold area that doesn't exceed -20F. I can't help any further with experience but those are the conditions you are faced with.

Also to remember, alot of JRM don't hold the red all season long. Some go green quickly, some hold it for longer. That is why you pay good money for "Bloodgood" and the other named varieties. Not knowing what cultivar you have makes it hard to know what to expect. Just try to differentiate. Green is OK, yellow and brown are bad.

Looks like what you have is a palmatum, they make a nice Bonsai or a good ornamental. The pics helped a lot.
Hellwig was right, depending on many factors you will get a variety of color--which I always found very nice and--they, with some exceptions, are very northern hardy.
Unless they have a dormant period it will die so keep that in mind. Easiest way to handle that is to allow it to go through a few hard frosts then put it in a bed of mulch (covering the top of the pot) under something like a large shrub or the south facing exterior wall (outside) of the house.
One thing they all suffer from is 'branch dessication' due to moisture loss in the winter--dieback of some new branches always happens. In the spring just cut the dead off and you're back in business.
I'd Google care of them and follow that.

Bob

So green is alright?
It's not dying?
Should I try putting it outside? It's been raining, but hasn't gone below 0C during the night

Not to leave you hanging there, I just don't have alot of personal experience yet. As long as the tree stays green, it is alive. Color loss may be natural for this maple or it may be a sign of not being happy but green equals living. Since we are into the growing season taking it outside should be fine for this half of the year but you are going to have to figure what to do with it in the winter. As with transitioning any plant don't just take it out and leave it. Try to start with a few hours a day, then a few more, then all day. Pending outside conditions and tenderness of plants sometimes you can rush this, sometimes rushing causes damage.