Short answer: Your system must use the same color profile as the print shop and be calibrated to the same spec. Everything else is just wild guessing. If your system is on a more generic level calibrated to a print spec (FOGRA standard) and the print shop provides reference profiles along with his proof being tagged properly with profile information, you should be able to get a reasonable representation, but not without any of that. And even then your print shop may still use some exotic stuff that only he can judge properly in his working environment. The better your own calibration, the better the match.
"Doesn't RGB reproduce all the CMYK color spectrum?"
RGB is a Color Mode and your specific monitor may represent a smaller or larger color space than specific CMYK spaces.
Any sets of RGB or CMYK values may have relative meaning (one pixel in one image being darker, redder etc. than another) but do not in and of themselves have a definitive meaning (in the sense of a certain color impression) unless they are refrenced to a specific color space.
And like Mylenium hinted at, if your screen is not calibrated (regularly and often at best) then you should not bother assessing sensitive colors on it.
This site has been recommended occasionally:
it appears your vendor doesn't understand the question well enough to explain it to you or it got lost in the translation
first, Ps (Photoshop) can indeed give you a 'faithful' monitor proof of your CMYK and RGB source images (if your monitor is properly profiled and you are using the correct ICC profiles)
second, if you made the Conversion to CMYK, then Ps should have already shown/displayed your colors on the monitor faithfully -- it is a completely different story if your printer Assigned and/or Converted to his profile (colorspace, CMYK) -- it is also a different story if you sent him out-of-gamut RGB and he properly Converted to CMYK -- none of which is clear in your post (or did i miss it?)
if you sent the printer CMYK, i think it it is pretty pointless for a vendor to upload another CMYK file for any number of reasons (unless he hosed your color and is trying to shirk responsibility)... but Ps should still be able to disply his version faithfully if you apply the correct Source profile
if you don't have confidence in your vendor, i would request a hard proof (a contract print you can hold in your hands) or find a different printer
if by "off line" you mean you are proofing the image in Photoshop
and by "on line" you mean you are proofing the image in a Web browser
you should probably just download his file and open it in Photoshop or Acrobat (and forget trying to "proof" CMYK in a Web browser because they are unreliable, if that is what you're doing)
in Photoshop (or Acrobat), 1) verify that they sent you a "tagged" document, and 2) you are using the correct source profile
if his doc is "Untagged" you definitely need to ask him what profile you need to Assign (to proof the colors faithfully)...
Thanks for all the info very helpful
By online I mean a web browser, and offline would be looking at the .tiff file I saved my design as (not looking at it in photoshop) in preview. I didnt bring the .pdf proof the printer sent me into photoshop, although thats a great idea. The design was created in CMYK gator, so no out-of-gamut business here.
The printer is gotprint.com and from talking to a few different people there, they told me "no one is trained to answer technical questions like that here." In which I replied...what? I wasn't able to talk to the people printing the cards themselves to get an intelligent repsonse to anything really...so I gave up and hit the forums. It doesnt seem like a company that mass produces this much material will really care about the little details enough to go through these things with customers. I guess kind of like eating fast food, or going to a nice steakhouse...quality. It's your choice
I'm not sure if they converted to their own profile.
I need to get in touch with callibrating my machine and working with proper color profiles. If anyone has some stellar links they use or recommend that would be splendid. Thanks for enlightening me mylenium and pfaff
if they indeed sent you a 'final' PDF [I] would look at it in Acrobat (using these settings):
if your monitor is good, Acrobat should preview your file okay without getting into a learning curve trying to verify Source profiles
Photoshop will also be able to open your PDF, but for several reasons (if the PDF file is untagged, for example) you might get unexpected results opening PDFs in Photoshop (including losing your vector info)...