Problems with an effect with avid and combustion

[travis]

Hi all
I need some help to make an effect.
I want to make a fake depth of field.
I have a clip (close shot of a character) and i am doing a colour mask with the background (in combustion). Then I put this new clip in the timeline V2.
In V1 i have the original clip which i added a blur effect.
So i aplied a chroma effect to get rid of the colour mask and mantain just the face of the character. The background mantain the blur effect.
I can´t do it properly.
I tried with diferent colour but all of them mantain a little but very visible line of colour (like if i use a red the edges of the character have this red line) I just clean it up when i put soft and gain from the effect editor (from either chroma effect or rgb effect) to the maximum values. But of course the face turns blurry too.
Is there any trick to achieve this well enough. Any info or general tips about doing this stuff...

I am very new to combustion. In fact i just started with it and the only reason to use it was for getting this effect. I could get after effects so if anyone knows how to do it with after and avid.
Or maybe it´s posible to do it just with avid.

I hope you can understand the message. My english is getting worse day by day

Thank

[MichaelP]

When you are rotoscoping on Combustion, don't use a color for replacement, but create an alpha channel for the area you want keyed out. That way you have more control over the edges of the alpha channel in combustion via feathering etc.

[travis]

Thanks Michael
Seems a better way to do it.
I ll be triyng during the weekend.