Athens has new underground system for the Olympics 2004. I thought that a shot of this would complete my vacation photographs. I manually duotonized it in PS to better achieve the grey and cold feeling these places always make me feel.
I developed my technique purely by trial and error, probably it's not the best one, but it works for me. The original usually is a colour photography... so I first have to desaturate it... sometimes I just use the saturation slider, other I go to the channel mixer and I select the most accurate one.
Then I decide what effect I want to achieve, I usually think of what highlights, midtones, and shadows colours I want. Finally I create some layers and combine them using different blendings to get the effect. You need to know the effect of each blending so you slowly go near and near the effect you have in mind.
In this particular case this image is composed of three layers. One uses colour dodge, the open pin light. The three tones in each individual layer are: pure magenta, pure cyan, and the background has a dark blue colour.
It's a long process combining opacities and blending options, and also sometimes tweaking colour balance of each layer in a "highlight", "midtone", or "shadows" bases.
But you have full control of the effect you want to achieve.
And... that's it. I know there are options for auto duotone in PS, and external plugins and programs to do it... but I find more fun and more control doing it manually :).