Now is the time to make your slime the color you want it to be. Add food coloring a few drops at a time, stirring it in between drops until you get the color you want.
The standard color for slime, of course, is green, but you don't have to stop there! If you want to make your slime more sickly looking you can add some yellow as well. Red slime would be good for the guts of the “mad doctor's patient” in your haunted house. Red and blue together will make purple slime. The color is up to you!
Once the color is to your liking, add the liquid starch and mix well. The mixture will start to thicken instantly. When it is thoroughly mixed your slime is ready!
Experiment! I made a batch of light purple slime, and then after I mixed the starch in I added a few more drops of red and blue and stirred it some more. Since the mixture had already thickened, the additional colors didn't mix completely and they made streaks of red and blue that looked like veins. Ewww!
Keep your slime in an airtight container so that it won't dry out.
Liquid starch was commonplace when I was a kid but now it is hard to find. There are various sources for it online, but the only major walk-in retailer that I could find who carries it is Walmart. Try your local craft store as well; the craft company Darice makes liquid starch. I tried some of this also and it, too, made good slime.
Credit for this slime recipe goes to Science Bob. His website also documents two other recipes for slime:
Method 2 uses borax instead of liquid starch. Borax might be somewhat easier to source; look in the laundry-supplies aisle of grocery stores and similar retailers.
Method 3 makes transparent slime using borax and polyvinyl alcohol (PVA). PVA is extremely hard to find, so Science Bob offers kits.
It occurs to me that it might be possible to make slime that glows under UV light by adding Vitamin B2 to the water first (refer to Becky Stern's Kryptonite Candy guide), but I haven't tried this yet.