RGB is the color process TVs and monitors use. Red, green, and blue light is mixed together on screen to make the full spectrum of colors. Alternatively, with CMYK, cyan, magenta, yellow, and black ink is mixed together on paper to make the full spectrum.
Unlike CMYK, which has a pretty logical percent system of mixing the colors, RGB is a little trickier. Values range from 0 to 255, where 0 is black and 255 is the pure color. When the pure colors of red, green, and blue are mixed together you get white (R 255, G 255, B 255).
You can use this tool to play around and see what colors different values make: www.colorspire.com/rgb-color-wheel
The very first experiments in RGB started way back in 1861 with photography where they layered three plates to produce a color image. But it wasn’t until 1938 that the modern RGB technology for television was developed.
Why should I care?
Main takeaway: When you upload your images from your phone or digital camera, they start out as RGB. But if you’re using those images in a document that’s going to be professionally printed, you should convert them to CMYK. This is probably the easiest thing to do in Photoshop: simply open your image, click Image/Mode and select CMYK Color. Save and you’re done.
Images look almost the same in RGB and CMYK but you might see some slight differences in really rich colors. Since RGB colors are made of light and CMYK colors are made of ink, there are bound to be some inconsistencies