luma keyer makes the alpha channel footage look grey(ash)

Hi. This question may sound stupid but I tried and can't figure out a solution for ages.

I downloaded a alpha channel snow footage which has all the snows in it in white. When I applied luma keyer, the snow looks grey in colour. I tried adjusting the settings but it doesn't seem to help. 

Does anyone know how come this may be? Thank you. 

If it has an alpha channel, you should not really need to do anything. But yeah, sometimes you need to set Blend Mode to stencil or silhouette Alpha. You should not be using a Luma Key for an alpha channel.

thanks for the reply. The snow footage stated that it has an alpha channel to it, but without a luma keyer, it covers up the main clip. perhaps somehow when I got the footage, the alpha channel was lost.

I will try the blend modes, I haven't thought about that.

Update 1: I tried using hard mix blend mode. It worked and the snow turned back to white. The other 2 that was suggested didn't work. Thank you again!

I suspect that clip doesn't have an alpha channel. What format is it?

I had another look at the clip. It has an alpha channel indeed. I removed the luma keyer effect and changed the blend mode to add and it worked fine! Thank you. 

The file is in .mp4.

MP4 does not support the alpha channel, that's why you had to use something to make the dark part transparent. If it had an actual alpha channel you'd need not do anything, pretty much. So no, you do not have an alpha channel in that file. What gives you the impression it has one?

I thought it had an alpha channel because changing the blend mode removed the black background. 

what file format would a video be in if it has an alpha channel in it? (I don't know as I only just found out that mp4 files can't have alpha channel)

With FCP users, it would be ProRes 4444. But long-GOP formats like mp4 normally don't support alpha channels.
Blend Mode has nothing to do with alpha channels but mathematically manipulating the chroma and luma value differences of the pixels of two images. A Blend Mode works on the selected image and the image just below it.