I tried this, it didn’t work

The paint made the water too blurry, and you couldn’t quite see what was inside.
You could experiment with food juices that you have in the fridge though, and that works. I ended up using cleaning liquid though and it doesn’t look bad and I was able to see what’s inside.I had a couple of cleaning liquids that were colored and I’m still experimenting there. I was able to find purple, yellow, and blue I had around. Anyways, I thought I would share this, since the paint didn’t work for me. Another alternative is also dish soap (like dawn diluted).