You can use black and white ink or many colors of nail polish.
This design is not limited only to nails, as you can also dye your hair in gradual tones from black to dark brunette or dark brown to blonde.
This design is also suitable for the eyes. Some people also dye their eyebrows because when the colors are applied to hair and nails, you can get a unified look.
You can create this gradient effect on fingernails using different nail polish colors. You can use two shades of black in the middle and two shades of white at the ends of your nail, or you can use many colors like black with purple, blue, or green tones; green with blue or purple tones; or white with a pink tone.
You can use any nail polish for this design. For example, you can use a solid black shade for the middle and white shades for the end. You can also choose black with purple varnish or blue with a green tone.
White nail polish is more suitable for the end parts of your nail because it will make your nails have less contrast. However, you can still use a dark color like black in that area if you prefer using darker colors.
If you want to get a more solid color of nails, you can use the ombre nail polish technique, but instead of using different shades, you will use different colors. You can paint three or four parts of your nails with three or four colors – black, white, and two more colors.
You should cover your nails with a black base coat because it is an opaque color that will give you a solid black color on the first coat.