you can do it manually:
After converting to grayscale, change the Hue/Saturation set to colorize. select a new color with hue. Use the Saturation and Lightness to adjust the saturation and lightness of the pixels.
how to make a monochrome image:
How do I change an image to Monotone, is there such a choice in LR?
I assume you mean Monochrome, black and white, and tones of gray or any other color. The details depend somewhat on your version of lightroom. In any case, I think of it as a two-step process.
First, you'll make a black and white with contrasts as you like it. Then, you can convert the "grays" to any color of your choice. Of course you can quickly create a B&W by moving the saturation slider fully left, but you loose the ability to differentiate different original-colors that have the same tone (brightness). Much better to work in B&W mode because that allows altering the brightness of individual colors.
I'll describe what I do in LR 4.
Select B&W, either in the Treatment or in the HSL / Color / B&W panels. Adjust all the basic sliders -- White Balance (it matters!), Tone, and Clarity, and the Black and White Mix sliders to alter brightness of different colors. Feel free to use the tone curve, if that's your thing.
When you are satisfied with your B&W image, go to the Split Toning panel and adjust the Hue and Saturation sliders to add color. Technically, for a monochrome, choose the same Hue and Saturation values for both the Highlights and Shadows.
Finish up by making whatever other adjustments you would make for an image.
This is actually a very easy to use and powerful procedure. Have some more fun and try using two different colors for the Highlight and Shadow toning. Of course, that costs you "honest monochrome" status!
Europe, Middle East and Africa