when are you infetious with a cold?

I think a cold is infectious for a few days before you are sick and then for as long as your symptoms last. Its a virus so anything up to two weeks.

I was told the first three days are the time you can pass on a cold. I have never tested this theory out of course, but it's what my very wise mother always believed.

Not sure how this works
but 2yr old GS seems to be a carrier, bringing colds and cough from nursery and passing them onto his little 1yr brother.
2yr old seems ok no symptons.
1yr GS has coughs and sniffles.
(The nursery always seems have children with colds and runny noses).

While you are coughing and sneezing...

It is an airborne virus with droplets containing the virus spread by coughing and sneezing. It can also be spread by direct contact e.g. kissing etc. You can inhale the virus or catch it from touching an infected surface. So you are infectious as soon as you start having these symptoms until you stop. The old saying "coughs and sneezes spread diseases" is accurate.