Convert grayscale value to RGB representation?

Conversion of a grayscale to RGB is simple. Simply use R = G = B = gray value. The basic idea is that color (as viewed on a monitor in terms of RGB) is an additive system.

http://en.wikipedia.org/wiki/Additive_color

Thus adding red to green yields yellow. Add in some blue to that mix in equal amounts, and you get a neutral color. Full on [red, green, blue] = [255 255 255] yields white. [0,0,0] yields monitor black. Intermediate values, when R=G=B are all equal will yield nominally neutral colors of the given level of gray.

A minor problem is depending on how you view the color, it may not be perfectly neutral. This will depend on how your monitor (or printer) is calibrated. There are interesting depths of color science we could go into from this point. I'll stop here.

The quick and dirty approach is to repeat the grayscale intensity for each component of RGB. So, if you have grayscale 120, it translates to RGB (120, 120, 120).

This is quick and dirty because the effective luminance you get depends on the actual luminance of the R, G and B subpixels of the device that you're using.

Grey-scale means that all values have the same intensity. Set all channels (in RGB) equal to the the grey value and you will have the an RGB black and white image.

If you have the greyscale value in a the range 0..255 and want to produce a new value in the form 0x00RRGGBB, then a quick way to do this is:

int rgb = grey * 0x00010101;

or equivalent in your chosen language.