No matter how well you calculate the quantities of paint needed for a job it is inevitable you’ll have some surplus…
This may not be a problem if it’s a colour you can use elsewhere but with so many colours available it’s likely you’ll be left with half a tin full of some obscure colour you’ll never use again.
Rather than just storing this paint away in the vain hope it will come in handy for something later (which is very unlikely) you could take the paint to your local council recycling centre.

You can search for local authority run recycling areas by postcode here or just search Google for ‘paint recycling centre’
Recycling Paint
If you take your old paint tins to a recycling centre you may wonder what happens to it?
In many cases water based emulsions can be filtered and re-processed into new paint; some other ingredients which can not be re-used in paint can used in the production process of some other material, such as cement or for fuel oil, so your old paint is very rarely wasted.
There are a few companies in the US who market recycled paint to consumers but in the UK there is, so far, only a couple of such enterprises.
Safely Disposing of Waste Paint
If you only have a small amount of paint left it may not always be practical to recycle it, you may be better off just throwing it away. And, assuming you are a responsible person, there are safe ways to do this.
Some paint retailers will also take your used tins and dispose of them properly. They may only take back tins you’ve bought from that store though.
To dispose of in the regular rubbish, the common advice is to leave the lid off so the paint solidifies first. This can take a while though and isn’t always practical.