Box dyes can be a cheap solution to coloring hair, but sometimes the exact color you want isn't available. For example, you might want a dark red shade, but you want an auburn red, not a mahogany. It is possible to mix a shade of brown with your red shade of choice, and it's not difficult. When buying the two shades instead of one, it's not twice as expensive. Use the color you will create twice. Dark red hair color can work on light and dark shades, previously dyed or not.
Choose your two shades. Stick with shades of the same brand and with a similar tones. For example, a warm shade of red goes with a golden brown; a cool shade of red goes with a cool brown. The box will specify whether the tone is warm, cool or natural.
Take inventory of the coloring supplies. A box dye typically comes with a developer, a color gel, a pair of gloves and a conditioner. With two boxes, you should have two developers, two color gels, two pairs of gloves and two conditioners.
Put on a pair of gloves, and open one developer and the two gel colors. Pour half of each gel color into the developer. Close the caps of each gel color, and set them aside. Place your thumb over the developer, and shake it until the contents are thoroughly mixed.