When designing for online or print, you need to be aware of the correct colour profiles. Basically RGB refers to digital work and CMYK refers to print work.
RGB (or additive color space) stands for Red, Green and Blue. They are the colours used by monitors to recreate all the colours you see when viewing anything digitally. You cannot rely on what you see on your RGB as each person’s monitor is calibrated differently. So a bright blue/green on your monitor, may look more blue/grey on another person’s monitor.
CMYK (or process/4 colour/full colour) stands for Cyan, Magenta, Yellow and Black (honest, even though it’s K). These represent the colours that are used in a modern day digital printing press.
The printing process uses a mix of these four colours to create all the other colours in the spectrum. CMYK is made up of lots of little dots to fool the eye into thinking it’s another colour.
Example from www.unisa.edu.au
When designing a logo you must design with CMYK in mind and not use RGB, unless you never intend to see your logo in print. I use my trusted Pantone Colour Bridge book which shows me swatches of colours and what they look like in Spot (single block colour) and CMYK version. This helps me ensure that what the client sees in proof will be as close as possible to the finished printed product.
Unless the client can afford to pay for Spot colour printing, I tend to avoid vivid colours (lime greens, neons etc.) on the spectrum. Generally a vibrant colour that you view on your RBG display will not be the same in CMYK print. This is purely down to the fact that CMYK just cannot recreate vivid colours RGB can so it would be wrong to rely the RGB colour space when designing any branding or colour critical print work.
As part of my logo/branding package I also provide clients with a style guide which displays the colours of the chosen colour palette along with all the values for each colour in their Pantone, CMYK, HTML and RGB references. This handy resource will assist in keeping the colours and brand consistent when working on any future online or print material projects.
It can be a minefield, but with the help of a good designer, it should be a completely pain free process. I hope that this article helps explain why a good designer is worth their weight in chocolate buttons.
This is only a very short summary of RGB and CMYK, if you want to know more, you can read up on it on wikipedia.