Sombré is a variation of Ombré. Ombré is the gradual blending of one color hue into another. Sombré is just a softer version of that gradation. In order to achieve a true Ombré there needs to be a difference of at least 4-5 levels on the color scale. On the hair color scale 10 is the blondest hair, 1 is the darkest hair. For a true Ombré, if the client's ends are a level 10 her roots need to be a 6 or darker. The Sombré color doesn't need to have that drastic of an effect. You can achieve a similar result with only a variation of 2-3 levels difference between the roots and the ends. This allows the client to try out a trend without having to commit to a drastic color process. Most of the focus will be on the ends while maintaining the depth and shine at the roots.
I use the balayage method of application for both the Ombré and the Sombré method. Pictured below you will see that i am using a paddle to help provide a backing as I paint the highlights onto the hair. The process of painting the color on in large sections allows for the ends of the hair to receive a lot of color, which is crucial is achieving a successful Sombré.