Get a bucket of water based paint. Decide what color to use. I recommend finding something that is dark or that is very light. Humanoids are likely to choose colors close to neutral gray to use on their homes and businesses to keep things calm. Go For Contrast. MAKE A FUNNEL and pour 3/4 of the paint into the empty extinguisher. fill a full bucket with water, AND USING THE FUNNEL,pour into extinguisher now containing paint. Close the beast aka seal the chamber. Tighten the top nut and shake vigorously.