In a separate small bowl (you can use the same one from step 2) mix together the water and food colouring and add it into the mixture, drop by drop. Be very careful because the water will react with the citric acid causing the mixture to bubble, just be sure to whisk well to prevent too much reaction and not to add too much water. I had one batch totally bubble over while it was drying because I put too much - oops!

Test the mixture out by taking handfuls and squeezing it together. If it keeps it's shape, you can proceed to molding. If the mixture is still too dry to stay together, add a couple more drops of water and repeat until the mixture does keep it's form. 

When the mixture is ready, firmly pack the powder into molds and let the bath bombs dry for at least 4 hours in the mold for best results. (I used a large silicone whiskey ice cube tray that I got at Dollarama. Alternatively, you can use sphere molds - I find ones with a diameter of 6.7 cm works best. This recipe makes three bath bomb cubes OR two bath bomb balls using these molds.)
Some fun scents to try out:

Lavender

Lemon + Bergamot

Peppermint

Peppermint + Cocoa

Orange

Strawberry

Vanilla

You can also decorate your bath bombs with dried flowers, tea leaves, icing decals, fine glitter or shaped sprinkles. To make the colour more pigmented, you can use mica but use it sparingly (start off with ¼ tsp). I personally prefer not to use these additions because it’s a bit of extra clean up afterwards (I'm just that lazy lol...).

I really enjoyed making these bath bombs and think they make really cute and affordable gifts.

Do you have a favourite DIY project? I'm looking into making some show scrubs next!