Are you meaning 15 stitches in multiple colours in the same row??? If so, you are referring to intarsia knitting. A video can be found here
under the 'How to knit with 2 colors at a time' heading. In intarsia, you have a seperate length of yarn for each block of colour in your knitting, and rather than tying off each time, the strands are twisted together at the colour changes.