I'd use Paintstix or Derwent colored pencils. For the Paintstix, use a stencil brush to scrub the paint onto the fabric where you want it a darker yellow. You can blend w/ the stencil brush so the edges won't be a sharp line. Derwent colored pencils (many YouTube videos) can be applied a couple different ways. You use a fabric medium or clear aloe vera gel to make the color flow and it will also make the color change to very vibrant. Heat set with an iron after it's dry. You can put the fabric medium or clear aloe vera gel on first and then color into it or the other way around. The pencils can be blended some by just layering one over another. Try it out first on some scraps.