On top of the liquid, add the flour, sugar, salt, yeast, and cubed butter.
Mix just until the dough comes together, but don’t overmix.
Set aside ¼ of the dough and transfer the rest to a lightly floured surface. Knead until the flour is fully incorporated. Shape into a ball, place in a large bowl, cover with plastic wrap, and let rest at room temperature for 2 hours.
Divide the reserved quarter of dough into 5 equal pieces. Add a few drop of different colored food coloring to each one.
Wearing food-safe gloves so you don’t stain your skin, work each color into the dough.
Shape the colored dough into balls, place on a plate, and cover with plastic. Let rest for 2 hours at room temperature.
Transfer the large piece of rested dough to a parchment-lined baking sheet. Shape into an 8x11-inch (20x27 cm) rectangle, cover with plastic wrap, and chill in the refrigerator overnight.
Transfer the colored dough to another parchment-lined baking sheet. Shape into 2x3-inch (5x7 cm) rectangles, cover with plastic wrap, and chill overnight.
Transfer the plain rested dough to a clean, lightly floured surface. Roll into a 16x8-inch (40x20 cm) rectangle. Pat the edges with a knife to straighten.
Place the butter rectangle at the center of the dough rectangle. Fold the top and bottom of the dough over to cover the butter, pinching the center seam and sides to seal.
Rotate 90° and roll out to a large rectangle, about 8x18 inches (20x45 cm). Flatten the edges.
Fold the top and bottom edges to meet in the center of the dough. Fold over once more, then freeze for 30 minutes.
Roll out the dough again to a large rectangle, about 24x8 inches (60x20 cm). Brush off any excess flour. Fold the dough in thirds, brush again, then freeze for another 30 minutes.
Cut the chilled, colored doughs in half lengthwise. Roll into ropes about 10 inches (25 cm) long.
Brush the ropes with water and line up together, in rainbow order or as desired. Press together. Lightly dust with flour and roll out into a rectangle roughly the same size as the plain dough.
Brush the plain dough with water, then lay on the colored dough sheet on top.
Roll out into a large, 13x21-inch (33x53 cm) rectangle and about ¼-inch (6 mm) thick.
Cut the dough into 10 triangles with bases that are about 4 inches (10 cm) wide.
Lay the dough triangles colored side down, and starting with the wide end, gently roll up.
Place the croissants on a parchment-lined baking sheet, seam-side down. Cover with plastic wrap and let proof at room temperature for 2 hours.
Preheat the oven to 400°F (200°C).
Brush the croissants with egg wash.
Bake for 20-25 minutes, until golden brown. Let cool on a wire rack.