First let me clear something out : there's no difference between fading out a white frame using the red rubberbands or a dissolve transition.
2)
Here's a 5s clip showing the flashes I have to put almost all long the video. I'll keep the second one (the flash which fades out like a wave) but I want to make something else for the first one (which is just a faded white frame) -- done, see #6.
3) I can't use clips instead of flashes because it's already done further in the vid but it needs something more (a flash).
4) Don't tell me about red or green frame flashes

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5) Thanks for the lens flare idea. Doesn't really fit the first flash I want to change but I used it for another part of my AMV.
6) Thanks a lot to OmniStrata ^_^. The level effect set to 0->255 over three frames looks quite like a white frame fading out but with a little something more... I'll use that.
7)
Here's a part of my vid using both white frame and level flashes. The first serie of flashes uses white frames and the second one level effect. Tell me which you prefer.
I like level flashes more because a white frame is already used for the lady-in-the-red-ball clip (I can't use level effect there because I made the scene black&white).