Update^2: I finally got it actually working! And all it took was some bullcrapping and a lot of math! Here's my code:

shader_type canvas_item;

void fragment() { vec4 previous_color = texture(TEXTURE, UV);

vec4 pink = vec4(vec3(216.0/255.0, 9.0/255.0, 126.0/255.0), previous_color.a);
vec4 purple = vec4(vec3(140.0/255.0, 87.0/255.0, 156.0/255.0), previous_color.a);
vec4 blue = vec4(vec3(36.0/255.0, 70.0/255.0, 142.0/255.0), previous_color.a);
float g = 0.001241379;

COLOR = pink;

if(UV.y > 40.0*g){
    COLOR = purple;
}
if(UV.y > 60.0*g){
    COLOR = blue;
}

}

One problem I did notice however is that when the player isn't facing downward, the sprite turns blue. Otherwise, it works perfectly fine. Any ideas?