Changing the color of "Film Burn" effect

It is not black, the film burn effect deals with the alpha channel of the layer. 

To actually give it color you need to:
Apply the effect on a black solid
Make a white solid and place it under the black solid.
Change the settings of the film burn effect to whatever you want.

Select both layers and precomp them.
Put a tint effect on the new precomp and change the Map White To Color

And you should be good.

Black is the only edge color. You can apply the effect to a white solid then add some color correction and blend modes or double up on the effect and use track mattes, but there is no one-click method of simulating the actual look of film melting