4 colour process means it uses all 4 inks (cyan, magenta, yellow & black). 4 colour process means the cmyk code has a value for each. you could do a straight run of nothing but flat orange and it will be the same price as printing your photograph is your orange has 4 values for the cmyk.
2 colour would be like c:0 m:100 y:0 k:50.
|