RGB and CMYK are the two colour models every designer will at one point work with. They are different modes of colours both used for their specific task. One is used for digital work and the other for printing purposes.
The acronym RGB stands for red green and blue. These are the colours used for digital imagery. A colour used in any editing software will be shown by blending a digital numeric amount of red with green and blue (ex. R255,G255,B51 = FFFF33 yellow). These colours all start with black, to which different colour ‘light/brightness’ are added to create the visible colours
The acronym CMYK stands for cyan magenta yellow and key (black).
(Ex. C 65%, M 0%, Y 65% K 63% = huntergreen) These colour schemes are only used for printing as they cannot be shown on most screens. CMYK is a subtractive, reflected light colour system meaning the colour starts at white “paper” which later on, coloured inks are added to subtract from the white.
Unfortunately RGB colours can’t be printed and CMYK colours aren’t correctly shown on digital media but editing software such as Photoshop and Illustrator try to convert the colours used from RGB to CMYK for easier.