Black & White
For budget plastic cards, a black and white print is the way to go. Just because it's cheap doesn't mean it can't be beautiful too. There are plenty of ways to make bold one colour designs eye-catching!
CMYK (Full Colour)
Similar to your home printer, printing in CMYK splits your artwork into four layers - cyan, magenta, yellow and black to reproduce an array of different colours.
Spot Colour (PMS)
Pantone is a system of spot colours, specialty inks that are used to ensure exact colour matching and a superior print quality. PMS colours are also used to print fluorescent and metallic colours, something that can't be achieved with a standard CMYK print.
PMS colours can be used in combination with CMYK or Black & White print. Just know that the more colours, the more pricy your plastic card job will get. For more information about Pantone Colours read this.