Welcome Eve Flickr has its quirks that get under my skin like over-sharpening. I haven't noticed a big color shift with my own photos though, are you converting to sRGB color mode (Perceptual) when you're saving for web? Or switching back to 8 bit color if you were working in 16 bit? These are 2 big things I can think of that affect how colors will get displayed and how different websites can interpret your color settings.
I don't convert to sRGB as Flickr does that anyway - I hate the way Flickr sharpens - so I don't sharpen anymore when I reduce the size for display there. Yes I always switch back to 8 bit as I can't save a 16 bit JPEG Anyway the blues are beautiful either way