Offset means “indirect” since in its dynamics through a treated metallic plate the design is transferred to a rubber and from there, to the support. If the plate were to print directly on the support, the definition would not be as good.
Each plate corresponds to a colour. With the printing in four-colour process (cyan, magenta, yellow and black) any colour can be reproduced except metallics, flourescents and some outside of the range of the four-colour process.
For these cases and actually to guarantee the reproduction of an exact colour there are the colours Pantone*.
*Range of colours defined by a number and acronym according to the surface in which it is printed: for paper C (coated) and U (uncoated), TCX (fabrics), TPX (decoration paper), Q (opaque plastics) y T (transparent plastics)…