1. I would divide my artwork resin into cups and color as desired using the Resin Obsession brand: https://shop.resinobsession.com/collections/resin/resin-obsession-artwork-resin
2. For the colors I want to be more free-flowing, I would add a solvent such as acetone or denatured alcohol. Start with a few drops and stir until you get the consistency you want. Don’t add more than 10% of the total volume of resin.
3. If you wait until closer to the end of the pot time of the resin, it will be thicker to pour, but you will have a harder time getting all the bubbles out.