Image Sequence Texture & Purple color

Ok, I’ll try to make my question as clear as possible:

  • My scene length is 100 frames (from 1 to 100).
  • My wet map sequence ranges from frame 57th to frame 100th.
    => In the first 56 frames, blender uses a purple texture instead, which messes up the material I’m trying to make.

Is there anyway to make blender ignore the first 56 purple frames ?(like, Is there any node I can use to map the purple color to black?)

Thank you in advance, sorry if this is asked else where.
And Merry Christmas everyone.