all colours when printing are done using a four colour process.
a spot colour is a single colour.
e.g. red, black, yellow, blue etc etc.
if you print say yellow without using a spot colour. it would use four colours one on top of each other to acheave this colour. this making the same job four times the cost to produce as if you where to use a spot colour.
spot colour ribbons cost a bit more for this reason.. kinda fly but thats the reason they know you save money using them so they charge a bit more. another down side to using spot colours is say you wanted to print a navy blue. you cant using spot colour! because you can only print the one shade of blue that is given to you.
same goes for light blue it would not work.
this means if you get a job thats not the same as the spot colour you are using. then its back to the four colour process.