I thought I had seen a thread somewhere that discussed the issue of using grayscale images for eyes / hair and then changing the colour by material settings - but I can't seem to find it.

Can someone point me in the right direction?

In the material room, add a HSV node between the main node and the iris texture map node. Set saturation to 0 to get a greyscale map for the iris. You can then change the diffuse colour to get different colour irises.

I have a set of recolorations of MayaStudio's Honey Hair in freestuff that uses that method, if you'd like to see how it works .

That's just the type of stuff I was after,
Thanks raven & Kalypso

You can also use a math_function node--a simple Add gives a greyscale. For some purposes, you might want to alter the transfer function, using Multiply to stretch or shrink the brightness range, for instance. The Component node can split the texture into RGB channels, in greyscale, and sometimes using one of them can work out better than the overall greyscale. It depends whether you want to emphasis colour shifts or brightness shifts.