The slime has fully dried after 2 nights. I tried out ways to colour it today, using watercolours, inks, and acrylic paints.
Inks were most successful in colouring it vividly, and maintains the puffiness of the slime well. With watercolours, the slime disappeared. Acrylic is fine too, but it should not be mixed with water or the slime will disappear as well.
Food colouring on top was also fine, and it seems the most ideal in giving a subtle and vivid colour. However I think I still preferred it when I added food colouring before the slime dried, as this was just as successful way as colouring it after, but a much shorter, time-saving process. I think next time, I will try colouring the slime before it dries, using a bit of yellow and blue ink to create a more accurate human flesh tone.
Currently, I am waiting for this slime to dry- it has fluffy wool in it so that the slime holds together more, thus becoming a more solid material for me to stitch on. I can cut strips and weave it too, after it dries completely.