First of all, Silkscreen and Offset printing do not refer to the same colour guides. Silkscreen printing is done refering to the Pantone (or HKS) color reference. On the other hand, Offset printing refers to the CMYK color reference.
Secondly, the two processes are used for different purposes; Silkscreen will be used for artworks with non-gradient printing, while Offset printing will be used for artwork with gradient colors printing (example: a photo picture)
Thirdly, the painting colors used for Silkscreen printing is thicker than the one used for Offset printing. Offset printing uses painting colors that are more liquid to get thinner grain and detailed definition.