Disable gradient in 3D lines, R

I’ve plotted multiple lines in a 3D line chart. The colours of each line are not dependent on any variable change, but on colours that I had to enter manually for each line, stored in a separate data frame.

I wonder if there is a way to disable what seems to be a default colour gradient? As in, I don’t want one colour smoothly blending into the other line in the picture, I’d like it to only be pink in august and july