Make sure you are working in sRGB color space in Photoshop, and do not use color profile embedding. Fireworks does not read color profiles, nor do most browsers.
Thanks for your suggestion Linda, but that is how my colour settings are saved and still the colours in the design look considerably different.
One thing I have noticed is though (as I dont often use Photoshop for this) when I 'export for web' the image then appears to match it opened in Fireworks perfectly. Therefore I'm assuming what you see on your canvas in Photoshop is not exactly how its going to look for web. I even screen grabbed a website there and pasted into Photoshop to test and it looks (colourwise) quite different. Whereas in Fireworks it looks exact.
My main concern is if I am being supplied artwork by someone who is designing in Photoshop, they are mocking up a darker red and wish to have a darker red. But when I take the item into Fireworks to complete the design the red is of lighter colour.
You've just stated that you get the same results when Saving for Web from Photoshop, so regardless of the software, you're getting the same result.
Are the files you are working on in PS using the CMYK color space? If they are, that will also affect the end result as everything must be in the RGB color space for web graphics. Fireworks will convert CMYK to RGB and things will change.
What is your Proof setup in PS? It' s likely its set to Working CMYK, and you also have Proof Colors activated.
You can change this to Internet Standard (sRGB) by going to View > Proof Setup. It won't change how things look for the other artist, but it should bring things in line for you.
Added to this is the fact that no two monitors are going to render the colors and brightness exactly the same. Type of monitor, its age, how long it's been running and minimal - if any - monitor calibration pretty much guarantee that what you see won't be exactly what someone else sees on their monitor.
I'm using PS/FW CS5 and it looks fine in PS, i have it set to web standard sRGB, and exported for web, opens and looks fine in PS, but in FW it looks dull.
Seems like Adobe didn't bother to put color profiling in FW and did away with ImageReady, so we're kinda stuck.
I'll tried making guides, doing the exports from FW to get the code and saving out individual slices from PS and replacing FW's image files, but FW is an utterly useless and cumbersome piece of garbage.
All I want is to design in Photoshop, slice and export. Why is this now impossible? I'm trying to make simple rollovers, but there is no system for it. Instead i have to mess with all sorts of states and can't even get a simple button rollover to work in a straightforward way.
Adobe, please pay attention and make something like Imageready again.
Until browsers read color profiling, there's really no reason for Fireworks to have it. Have you tried doing CSS rollovers in Dreamweaver?