Adobe's Photoshop software is virtually limitless. There are stripped-down versions (and imitations) with ironically clumsy "user-friendly" interfaces often packaged with digital cameras, new computers, etc. Ulead(?) PhotoImpact comes to mind. Windows has a built-in Image Editor (not a very good one, unfortunately).
To go from full-colour to grayscale in Photoshop is easy as pie.
To colour a grayscale image is entirely different... there is no colour information contained in a grayscale image, only information about the brightness of each pixel. There is no way of "rescuing" the colour in a grayscale image.
To colour a grayscale image, you have manually tell it what colours to apply, and where.
It's very easy to tint a grayscale image-- like laying a colour transparency over a black and white photo. You can easily make it look like an old-west, sepia-toned photo, for example, but to apply full, real-life colour would involve painstaking "hand-painting," and probably wouldn't look very realistic.
But any half-decent image editing software should let you play around with the colours and tint the entire image, at least.
Good luck![This message was edited by Dwayne Martineau on 18 January 2005 at 03:07 PM.]