adobe photoshop - If I make a design in RGB mode and then change it to CMYK how will it look when it's printed?

I'm making a business card in Photoshop and it needs to be in CMYK mode. Now, I'm using a logo on it that I previously made for the client in RGB mode and when I put it into the CMYK mode business card design it desaturates it a little. I'm trying to get my head around whether or not it will print how it looks on my screen (ie desaturated) or how it looks when it's in RGB mode? Because, the colours aren't accurate in CMYK mode so if that's exactly how it will look when printed I will need to adjust the colours to look like they do in RGB mode. Sorry if that doesn't make sense - usually I only design logos and I've never run into this problem before.

Answer

Firstly, when you change color modes, you should use Photoshop's Edit->Convert to profile function. This will allow you to map the colors to the new profile in the least-obtrusive way. This should prevent the logo or other asset from noticeably changing colors.

Secondly, the reason people do print designs in CMYK is precisely because it allows them to work in the same color space as the printed result. However, you still need to have 2 things for truly accurate color reproduction:

1. Your monitor has to be calibrated properly. There are various ways of doing this, but usually the best way is using a colorimeter to calibrate your monitor and generate an ICC profile that can be used to accurately map your monitor's output with other color spaces.


2. Your printer has to be calibrated properly. If you're using a professional printing service, then they should have this part handled.


3. Ideally, you'd also have the ICC profile of the printer/paper you'll be printing with.

If you can get the ICC profile from your printer, then you can go to View->Proof Setup->Custom... and choose the ICC profile you received. Otherwise, the best you can do is soft proof on "Working CMYK".

Though, to be safe, you should also try to get a hard proof or contract proof to make sure things turn out exactly the way you want, especially if it's a large print job for something as important as a business card.

Note: Some printing services prefer that all files are kept in RGB. For instance, Blurb—the self-publishing service—requires all images and print files to be sent in RGB format. Perhaps this is because they also create e-books, or perhaps it's to simplify the process for end-users, but that's how they do things. And the ICC profile they provide for soft-proofing is an sRGB profile.