Images converted with Imagemagick and the following command:

c=0; for neg in true false; do for grad in 20bd01 0077aa 0e2843 e90113; do for i in  {Mons\ 2,spaceship,Monsters}/*.png; do let "c++"; $neg && opts="-negate" || opts=""; convert -size 48x48 gradient:white-#${grad} \( "$i" -trim -resize 48x48 ${opts} \) -gravity center -composite out/${c}.png; done; done; done