Positives were used so that the proof which was either a chromalin (kind of like a dye transfer) or a colour key (four coloured, CMYK tranies registered to create a colour composite ) could be matched and adjusted to the the original positive. The positive could also be used to do a press proof but not for a magazine (unless you bought a whole bunch of pages). Postives were wanted because it was a good 'go by', and was the source of the separations. Positives were also preferred because there is too much scale (the best printed page in a mag is probably 3 or 4 stops and a good neg can have 11 or more), far too much variance in interpreting a neg and separating a neg to match a print is painful (making a separation from a print is easier, but the quality is substantially lower).
Web presses intended for full colour printing will have 4 or 5 heads/towers and may have the same on the bottom so that the mag gets printed front and back complete in one pass. Spot colour for a magazine is generally done by mixing CMYk, unless they are having a special on a specific colour like a metalic.
watch this. its a very simple process, but in order to get it going right, it takes experience, and lots of knowledge
actually, this one is better.
Thanks for the replies peeps,interesting stuff