CMYK
CONVERTING RGB COLOURS TO CMYK
RGB = Red, Green, Blue. CMYK = Cyan, Magenta, Yellow, Black.
There are two main colour gamuts that you will encounter; three colour RGB for screen, and four colour inking CMYK for print - and they are worlds apart in how colours are reproduced or handled. The transmitted light you see direct from your screen simply cannot be replicated by the reflected light your eye receives from ink on paper to the same intensity. All files MUST be converted to CMYK for print so there are four colours now to consider. Because of this conversion you will find that some colours will be lost or muted. Blues and greens especially are harder to print, see below.
THE METHOD (try and have two copies open, with one kept in RGB, so you can see what changes are needed)
With an RGB image open in Photoshop, (flatten it if needs be), go; IMAGE - MODE - CMYK (don’t worry about any colour profile warnings, as this is another issue). You may notice a colour change. If it needs adjusting then go IMAGE - ADJUSTMENTS - SELECTIVE COLOUR. This allows you to change each individual colour by subtracting its opposite. For instance with Blue, you take out Yellow whilst adding Cyan. Reds, you would take out Green. For Yellow you take out Blue and so on. When saving, use; FILE-SAVE AS - Not Export. If you do use Export make sure you UNCHECK the Convert to SRGB box! otherwise it will still be an RGB file at the end of the day.
Note the subtle differences between these RBG and CMYK images? it may not seem much, but it’s still transmitted light, so when it’s printed on paper and viewed as reflected light, you will lose another 15% of its intensity, and it may well end up looking a muddy blue/grey.
Note the difference in the blue sky here? plus the green is muted. This shows us that the ink on paper process can’t replicate the intensity of an RGB image that you see on your screen. So once you convert, you HAVE to adjust the blues and the greens. The trouble is if you don’t convert to CMYK the printer or designer will do it for you, but they may not correct the lost colours and you may not like the result. And you will probably blame the printer!! But it’s not their fault. The responsibility rests with you. It’s your work, so make sure that when you send your file to the printer you have considered the above.
