You can use both "den" and "det" here
"Du ændre det fra gult til rødt". Did you remember to add t to the adjectives?
Edit: i got one af those build your answer asignments and had to build "du ændre det fra gul til rød"
i don't get when you would say that
if i'm talking about the light (lyset) i would say "du ændre det fra gult til rødt" if talking about the color (farven) i would say " du ændre den fra gul til rød"
what can you talk about, where you can say "du ændre det fra gul til rød"?