I have a room painted in red
I used what I thought was the same red paint to touch up
However it is much darker and has left big marks
How can I lighten the red colour ?

You probably would have the best luck priming over the spots with KILZ or a pigmented sealer. Give it a couple of coats. After it's dry cover it with the red that matches. You may need to mix your own touch up paint and test it in an spot hidden by a picture or something. Start light and darken the color a tad at a time. Then paint over the test spots.

I agree with westnlas regarding the Kilz primer. You can scrap off some of your red paint and take it to a paint store for them to match. You may have actually used the correct red paint but the original walls may have faded, that's why the new red appears darker.

If it was a spot that you patched, the paint probably looks darker at the patch because it needs another coat. That is why your being told to prime the spots first, which is a good idea. If it still doesn't match after priming, painting the whole wall will probably be faster than trying to get the paint to match.

Fix spot halogen lamps to the wall and keep on isde of the wall white