Hey! It is used mostly for the sake of being lightweight. You can easily change it to another variable such as the object_index, sprite_index, or whatever you fancy. To find GML raw color values i generally use https://chrisanselmo.com/gmcolor/. If you want to change the image_blend in the room, double click an object and there should be a rectangle indicating the image_blend color. Double-click that and you can pick one that you like. Let me know if you have any more questions!