Adds the values in add to the rgb-values in every color in sprite_colors and then returns the faded palette (the values in add may be negative).
palette must be a length-256 sequence of length-3 sequences, specifying a palette.
sprite_colors is a returned sequence from get_sprite_colors().
add must be a length-3 sequence of integers.
delay is a value specifying how fast/slow the fading should be done (3 is a good value).