1 Maybe it’s perception. Check with someone who also witnessed it. Ask them who they think came up with what. If they confirm what you say, your colleague knew he was stealing the idea. Tell your witness who came up with what and when, and see what the person says. Maybe it’s unfair or even outrageous that the colleague stole your idea, maybe the idea isn’t that good anyway. Either way, this will give you the motivation / justification to take it further.
2 Leopards don't change their spots. The thieving colleaugue is not to be trusted and given the opportunity will unfairly take advantage of you or anyone they perceive as a pushover if it benefits them. This is not your friend, or rather, only as long as it benefits him/her. You could ask them why they portrayed it that way, but you are not dealing with an honest person, so I wouldn't expect to get anything out of that. Unfortunately for you, these people have worked out that it can benefit them to take advantage of people who they know will not make a fuss ( aka: a pushover). Tell them that they gave a good presentation on the idea, but that this is your idea and that you are glad they like it, but cannot accept that they pass off as theirs. See what they say, unless they say ‘oh yes, my bad, let me correct this’, proceed to ignore it since the person cannot be trusted.
3 Inform your manager that this is your idea and that you are glad they like it, but cannot accept that your colleague passes it off as theirs. It is part of their job to ensure fairness within the team. By not telling them, you would be preventing them from doing their job as well as they could. Tell them that you do not want to make a fuss, but that it is a matter of principle and also, that you want your manager to be aware of the kind of person you are dealing with. There might be previous occasions this person has done this that you don’t know about. Either way, if nothing is done, this will happen again. To clear the water, offer to send a recap of the idea in written, and almost as an aside, highlight who came up with what part and when. Maybe you can credit your thieving coworker for the communication of the idea, as it would allow them to not completely lose face, which might invite some kind of retaliation.

Good Luck.