I have a black and white image, a scanned signature, where I want to change the colour of the black lines to blue (or red or something else). How do I do this? Preferably with freeware (I have Mac, Win7 and Linux, I prefer the Mac).
There are several ways to do this in GIMP, or in any other raster image editor (Photoshop, Krita, Paint.NET, etc.) implementing similar features. Here's one way to do it, demonstrated on a famous example:
Scan and crop the signature:
Convert it to grayscale, and adjust the levels to make the background pure white. If necessary, clean up any specks of dirt or other unwanted details:
Pick a nice shade of dark blue (or whatever color you want), and create a new layer filled with that color. (If it comes out dark gray instead of blue, change the image to RGB mode and try again.) Move it over the signature layer, and change the layer blending mode to Screen:
Admire the result:
Of course, there are several other ways to do this, as well. For example, you could:
put the signature layer above the color layer, and change its blending mode to Screen instead;
apply the signature as a layer mask on the color layer;
use Color to Alpha to make the black areas of the signature transparent, then position it over the color layer;
just use the built-in Colorize tool.
I'm sure there are other methods I've left out, too.
All of these methods have their advantages and drawbacks. For example, the Levels / Curves and Colorize tools are quick to use, and give you a single colorized layer without having to merge layers, but they're not so convenient if you already have a specific color picked out, and they don't allow you to use more complicated textures instead of just plain solid colors.
On the other hand, the Curves tool (and also Levels, to some extent) allows non-linear color adjustments, which can let you simulate very dark inks (that look almost black when applied densely, but show some color on the edges and in thinner strokes) better. That requires a bit more skill to get right, though.
Meanwhile, the Layer Mask method gives you a blue-on-transparent layer instead of blue-on-white, which might be useful sometimes. (But do note that you can achieve a very similar effect by applying to Color to Alpha to the finished blue-on-white layer, anyway.)
Thanks to the inspiration from the comments and answers above I downloaded Graphic Converter (for Mac) and played around a bit. Using the following settings in the dialog Picture -> Brightness/Contrast/Hue the black line become blue
Graphic Converter is not freeware but there is a 30 day demo period and that was enough for me in this case.