Hi and welcome,
To make a vegetable dye, you cut up or puree the vegetable/plant (beets, carrots, and red cabbage work well), and add a little water to thin down the puree or to extract the color. Let that sit for a day so the colors are extracted. Then you strain off the water, such as by using cheesecloth or a plain cotton muslin fabric. Then you take that liquid and set it outside covered with a cloth so bugs don't get in. This is to evaporate away some of the water.
Check on it every week or so. After it is half of the original volume, it should be a little thicker, and ready for use.
Add this before rennet and stir thoroughly. Add as much as you need to get the color you want, from a few drops, to several tablespoons.