The original colour should be Polychromatic Blue ... but that is a colour that is hotly disputed.
When Dad initially did the bike, he re-sprayed it with Ford Cosmos Blue. That was as close as he could get buying paint from Halfords across the counter. That is a bit too 'blue' ... so I have followed in the same tradition, and hunted around for a near colour available from the mass market without having paint specially mixed, or paying the ransom RS Paints charge. Especially since hte Old Man re-sprayed his Thunderbird with their Old Gold paint, and it;s the wrong colour. He owned a 1956 T-Bird in that colour, and as soon as the paint hit the metal, he knew it was wrong. So, while their paint is decent quality, it's not necessairly 'right'.
The actual paint I have is a Hyundai colour....It's a little less blue, and I quite like it. Decent etch primer, some filler primer, one hell of a lot of rubbing down...and a few coats of paint plus clear coat. It's all rattle can, but it has come out quite nice and shiny.
edit: You can see the last bit of Cosmos Blue on the bath-tub side piece behind the bike.