@Ahmed Khaled What do you want to do with the thin film colors? The ramp float it's only for remap float information (0-1 values) not rgb values.
Maybe other node works for you. And also, remember that the thin film shader will be removed at some point. You can recreate it with a ramp_rgb shader.
If you want to use a custom ramp you'll have to use something other than the thin film shader. The thin film shader is designed to produce colour values based on the thickness of the film and accepts values in nanometres.
You can kind of fake it by combining a noise and the facingratio, which you can then pipe into your custom ramp coordinates.
Was doing a similar thing the other day.. use facingratio to drive transmission colour and cutout opacity, each via their own rgb_float.. (if this is what you mean?)