Mono,Duo and Spot . Duotone is when an image is printed in 2 spot colours. this can be any to colours. This printing method can produce very different effects depending on the colours used and easily done on programs like Photoshop and InDesign. You can use more colours which are know as Tritones for 3 colours and Quadtones for 4. . Spot colour is a specially mixed colour opposed to a result of CMYK or RGB. This can be usefull for a designer wanting to produce say a campaign for a company and wanting to use a specific colour all the way through. the colours can be put into the printers to create consistancy within prinitng. This technique is extremely popular with photographers wanting to highlight part of a black and white photo. . Mono is only using one coloured ink and the stock its printed on. It doesn't have to to one tone the ink can be used at a percentage such as tints of the colour used. This creates a tonal image. Black is the most common colour used for monoprints but others can be used also.