Forgot to remove Red filter from taking lens after switching from the B&W to Velvia 100.
I think you may find this difficult. A red filter is a low pass filter. It passes light frequencies that are long (red and infrared) while absorbing higher frequencies (green, blue, etc.). IOW, your film likely recorded very little green and hardly any blue. And if it's not on the film...
But... I've been wrong before. One way to proceed might be to scan a frame that has what you know to be essentially white (specular highlights, white concrete, etc.). This will allow you to pick out the red color from the filter as if it were a "mask" over the film. Delete the mask (no, I don't know how, never done it myself, but I know it can be done) so that the white parts become white again. What's left should be all that's recorded on the film. Go from there.