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A few different bright colors will give you the best results. You can use up to four different colors.  You can select a set of colors to create an ombré effect. If you pick colors that are a similar hue but with different shades, you can fade from one to the other. You can also go from one hue to another that's next to it on the color wheel, for example yellow to orange.  Other types of string, such as yarn, can work for hair wraps. Just make sure whatever material you're working with is colorfast. You don't want the dye to run the next time you shampoo your hair. It should also be fairly thin to keep your hair wrap from being too bulky. Thinner braids work better for hair wraps than thicker ones. Make sure your braid is very tight.  Picking a strand closer to your face will have a bigger impact than one further back. If your hair doesn't hold braids well, you can secure it with a small hair tie at the bottom. If you already have your hair in braids or dreadlocks, you can add a splash of color by selecting one or more strands to wrap individually. You will need four separate strings that are roughly two- to-three times the length of your hair. Remember that it's much better to start with too much thread than too little. You can always trim off the excess at the end, but you can't add length to string that's too short. You can always position the knot lower on the braid if you want your hair wrap to start further down. Overhand knots are the simplest knot you can tie:  Make sure all four strings are parallel and bunched together so they can be treated as a single string. Loop the strings behind the top of the braid. Make sure that your braid is positioned in the center of the strings so that each end is equal. Take the right end and loop it back around the braid to the left. Start by crossing over the front of the braid and pull the end under and through the loop back towards the right. Pull both ends in opposite directions to tighten. Make sure your knot is very tight when making a hair wrap.  Once the knot is complete, each string you started with will be divided in half. Before starting your wrap in earnest, you'll have twice as many strings to work with. For example, if you began with four strings, you should now have eight individual strands hanging from the base of your wrap.

Summary:
Get some embroidery floss. Pick out a small lock of hair and braid it. Measure out and cut appropriate lengths of thread. Tie the threads around the top of the braid using an overhand knot.