First, make your challah dough. Prepare the yeast in a large mixing bowl for a stand mixer by whisking it with warm water and 1 teaspoon sugar (that helps activate the yeast). Let stand until it foams and puffs up, about 10 minutes. If it doesn’t get foamy, your yeast is either bad or the water was too warm or cool. Try again!
Using the whisk attachment for the stand mixer, add the remaining sugar, eggs, oil, and honey. (You can use a whisk if you’re doing this by hand.) Gradually add 3 cups flour and salt, either using a hook attachment with the stand mixer on medium speed or a spoon and your hands until the dough begins to pull away from the sides. Dough should still be slightly sticky and soft. Add more flour as needed if the dough is very sticky. You may not need all the flour. You will knead the dough more when you add the color so you don’t have to knead now.
Then divide your dough into six even pieces. I used a food scale to weigh them out.
Flatten out each piece and put some gel food coloring of each color of the rainbow in the middle of each one. Then mix until your dough is dyed! This will take a few minutes for each, so be prepared. Add more color as needed, and wear gloves if you don’t want purple hands.
Place the dough in a bowl greased with oil and cover. Let dough rise in a warm place until it has at least doubled in size, about 2–3 hours. I put mine on top of an oven heated to the lowest temperature. My colors stuck to each other a bit, so if this bothers you and you like doing dishes, put them in separate bowls.
Now time to braid! You can make two smaller challot or one large challah. If you're making two, divide each color in half.
Take one of each color and make six even strands. If you are having trouble getting them to roll out, let the gluten relax and try again. Line them up in rainbow order and pinch at the top to secure the end. Then take the purple strand on the right and weave it to the left over two strand, under one strand and over two strands. Repeat with the blue strand, then the green, etc. until you reach the bottom. Secure the other ends together and tuck them under the challah. Repeat with other challah.
Then let your challah proof again on a parchment lined baking sheet, lightly covered, until doubled in size and appears light and fluffy, about 45 minutes or so (exact proofing timing for challah will depend on environmental conditions.) Pre-heat the oven to 350 degrees F. Whisk the last egg yolk with 1/2 tablespoon water and generously brush over challah bake for 30-35 minutes at 350 degrees, rotating pans halfway through. If the challah start to brown too fast, cover with foil until done.