Tone a grayscale image in Camera Raw

Use the controls in the Split Toning tab to color a grayscale image. You can add one color throughout the tonal range, such as a sepia appearance, or create a split tone result, in which a different color is applied to the shadows and the highlights. The extreme shadows and highlights remain black and white.

You can also apply special treatments, such as a cross-processed look, to a color image.

Select a grayscale image. (It can be an image that you converted to grayscale by selecting Convert To Grayscale in the HSL / Grayscale tab.)
In the Split Toning tab, adjust the Hue and Saturation properties for the highlights and shadows. Hue sets the color of the tone; Saturation sets the magnitude of the result.
Adjust the Balance control to balance the influence between the Highlight and Shadow controls. Positive values increase the influence of the Shadow controls; negative values increase the influence of the Highlight controls.