what's the difference between Event and Sequence?

An event is something that is set to happen on a specific turn, such as a target being placed or nuke triggered or whatever. (Or before the game starts, aka Turn 0.) This happens before the player gains control of their worm.

Whereas a sequence starts during the turn and goes through a list of...well, events for lack of a better word, until told to stop. So, it might drop five crates on the map, then reach "Stop sequences until no marked crates are left." or "Pause sequences for X amount of time." When these criteria are met, it continues on down the list. There are two sets of sequences, blue and red, which act independently of each other. (That is, a "Stop sequences" command in the red sequences will only stop the red ones.) If someone wanted to make say, a training mission which would go on indefinitely, then you can end the set of sequences with "Restart from beginning."

(Oh, and you can set events that pause/resume a set of sequences.)

This is based on the assumption that W:A missions work the same way as WWP ones do. Hope I helped. :)

yes, you helped
but "red" and "blue" for sequence are somehow related to team colors?

yes, you helped
but "red" and "blue" for sequence are somehow related to team colors?
Nope. Grim only called it this way because that's how the WWP mission editor differs between the two, it's basically "Sequence list 1" and "Sequence list 2". As far as I'm aware, the game can only handle these 2 sequences, and you can't add in any more. (take this with a pinch of salt, I've never tried "hacking" the game to handle more than 2)

WWP makes use of it in some multiplayer missions by having one team shoot crates and the other targets, one sequence will check to see if a crate has been destroyed while the other will check for targets, for example.

It might be worth downloading the actual WWP mission editor from the official WWP site, because although you won't really be able to make missions with it, you can see how things work by poking around the interface (I'll also assume that there are a few minor differences in WWP added in for Wormpot and the mission editor, so maybe not everything will work identically).

wwp mission editor doesnt want to work if wwp is not installed.

If I remember correctly,

Events: can only be triggered before the start of the specified turn.

Sequences: can only be triggered during a turn, and can be delayed for a specified length of time. Also, I believe all sequences are repeated when the last one is executed, while events only occur once.