I'm working on a branding project and having my background in digital disciplines I don't have too much knowledge about color systems.
Currently I'm trying to figure out what are the correct color values in PMS, CMYK and RGB. I did some research and I'm starting in Illustrator (in CMYK mode) with a PMS color and converting PMS to CMYK is pretty simple.
PMS 343C ->
C:98 M:0 Y:72 K:61 ->
R:0 G:82 B:57
But when I compare those RGB and PMS values in Photoshop side by side, they don't match. I get an RGB value of
R:0 G:86 B:67 for PMS
343C with color picker tool.
In agreement with DA01, it is best to use the Pantone Bridge to connect the dots, but what you are experiencing is that your "Edit/Color Settings" are probably different from Illustrator and Photoshop.
It is best to set your master color settings in Adobe Bridge and save them as a preset, which then makes all Adobe programs use the same Color Settings.
If you study color management, you'll discover that conversion is tricky. It doesn't just go from RGB to CMYK or PMS. First you have to convert CMYK to LAB using whatever profile Illustrator is set to and then to RGB with whatever profile Photoshop is set to. LAB is "device independent" meaning you cannot tell a printer with CMYK inks to "print 24L 189a 70b" (it wants to know how much cyan to lay down). Theoretically, every Lab value should correspond to a very exact color in the physical universe, in very exact lighting conditions. But unfortunately the LAB conversion isn't perfect, so if you convert CMYK back to RGB, you get a different value. At least the conversion in one direction will always come out the same (when you use the same profile and settings). The French are always researching further into color. Maybe one day they will figure out a better model than Lab.