Colour change in Photo Shop CS2 for Windows

If I have a picture of for instance a blue machine, then how do I change the colour into green not loosing the play of colours and shadows ?

One way is, make a selection of the area you want to recolor, on the layers pallette at the bottom hit the new fill or adjustment layer button, select solid color from the list, pick your new color, change the layer's blend mode to hue.

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If I have a picture of for instance a blue machine, then how do I change

the colour into green not loosing the play of colours and shadows ?

There are several ways.

Try playing with a Hue/Saturation adjustment layer

Or curves

Or channel mixer

etc...