Orient BlueThe stamps were issued by decree of the Minister of Hacienda (it would encompass the Inland Revenue Service in most countries), dated 14 June 1976. The order specifies the printer, size, colour, and even number of stamps per sheet.
Below is an extract of the order. The official colour is Orient Blue (azul oriente). The first stamps must have been printed by the date the decree was issued. It is likely the colour had been chosen by Correos and identified by FNMT that printed it by this time.
As Roberto's post evidences, the printer was unlikely to mix up the exact same colour for every printing. So 'shades' exist. However, it is likely the printer mixed up an ink that qualified as Orient Blue. - The succeeding series also was issued by decree. -
Interestingly, the FESOFI website mention a decree of 11 June. 1976. Although they are aware of a decree, still, they copy Edifil's colour and get the date of the decree wrong.

@Germania: Thanks, I was looking for the colour mentioned in the Michel catalogue. With a colour key, you are likely to get into the issue of shades. I was trying to stay away from that subject.
Only one colour was used. The portrait was printed in the same colour as the background. 'Shades' in photogravure do not just occur by mixing in black or white. A deep blue stamp supposed to be blue can be exactly that; printed from a cylinder etched deeper.