If you want to add new colorways to your collection, there are several options.
You can source stock fabric that comes in a range of colors. You will have to choose from the colors that are offered, but you can generally get lower minimums.
You can get fabric PFD or PFP (prepared for dye or prepared for print) and then get it dyed or printed in the exact shade of your choice. The fashion industry uses Pantone TCX colors (the cotton swatches) to specify color. When custom dyeing, there are usually minimums you need to meet for each color.
You can also cut and sew your designs in white and then garment dye the final pieces. Garment dye has a certain worn-in look and is not suitable for all types of fabric and garments, but is a good option for having more colors without having to invest in multiple colors of bulk fabric.
What method of adding colorways to your collection have you tried?