Setting your artwork to CMYK color mode
Since printing processes rely on the CMYK colour model, you must always set your artwork files to CMYK colour mode. To read more about CMYK and RGB colours, check out this article.
Of course, it is always best to set your canvas to CMYK mode from the very beginning. However, if you accidentally created your artwork file in RGB colour mode, you can follow the steps outlined below to convert it to CMYK mode.
Since changing the colour mode in your file is permanent (i.e. the change cannot be reverted), you might want to preview it in CMYK before the colour conversion process. To do so, click View > Proof colours in the top menu
The name of the tab should now show you that you are viewing your file in CMYK. Be aware though that at this point the colour mode of your file is not yet converted; this is just a preview. To change it permanently, you must follow the steps below.
When you are ready to change the mode of your file, navigate to Image > Mode > CMYK colour from the top menu:
You may see some warnings appear after making this selection. if your file contains smart objects, a pop-up notice will inform you that changing modes can affect the appearance of smart objects, and ask you if you want to rasterize. Select Don’t Rasterize.
After that, the program may warn you that changing modes can affect the appearance of layers. To keep the same appearance, click “Merge”. If you’re willing to take the risk, you can click “OK”.
A final pop-up notice informs you which CMYK profile will be used for the conversion. You can simply click “OK”.