color to alpha

[trandoductin]

need help python plug-in
I am trying to help someone on my forum to automate some steps
and one of the steps is colortoalpha
so I used
pdb.plug_in_colortoalpha(image,active_layer,color) but it isn't producing the same result as doing it from front end.

but if you do it through the front end there are two other settings one of which is the transparency threshold.

How do I perform the same settings as the front end specifying those 2 extra settings in addition to color?

Thanks in advance.