If it's a really fiddly job, you might want to use photoshop instead, I know it can do a colour replace, Fireworks doesn't have this feature, as it's basically a tool for building graphics for web or flash applications, not a photo editor.
The wand tool will select colours of a similar hue, that are touching, if you have yellow spread right through you pic you'll have problems with this.
What you could try is to use the levels control within the filters drop menu - although you can only select red, green or blue, you might be able to re-colour the yellow without effecting your picture too much.
You could also experiment with the colourise in the hue and saturation, both of these do change the whole pic though.
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Sometimes, when my code just won't behave, I take it outside and make it listen to britney spears music, and when it comes back it's really well behaved. I wonder if it's suffering from post tramatic stress syndrome now..