Trev’s suggestions are probably the easiest and safest if you can find something that matches what you want.
You could also perhaps use
thin pre-stained veneers if you’re more adventurous.
Maybe two different colours layered to get a step with the colour change (depending on headstock thickness and machine head thread length), rout a step to glue in a shaped red veneer, or jigsaw puzzle the two colours in the shapes you want so you have one flat piece to glue on the headstock. Something like Grant did
here, but the more complex the design or layering the more difficult it is to keep it positioned perfectly when glueing. Even just gluing multiple strips of thin veneer along the edge of the headstock may get you close to what you want. The benefit being it is far easier to strip off a veneer than glue back on pieces of a mangled headstock.
If your routing skills are up to making a channel along the edge of the headstock then red and red pearl plastic binding could also be an option… but the ‘chance of disaster’ factor is high.