Ok, quicker than I thought, but I can only find one - The Twins at St Clare's Granada books, 1967 edition) is a red dragon. Later editions are just called dragon books, with no different colours. It doesn't give an age range for the colours, but there is a list at the back of Green Red & Blue Dragons.
In Blue dragons there are only five books, all by Enid Blyton - Red, Blue,Yellow and Green Storybooks, and Eight O'clock Tales.
The Red Dragons listed are also mostly Blyton - the 'Five Find-Outers' series, St Clare's & Malory Towers, but there are also The Coral Island by RM Ballantyne and Knights of the Cardboard Castle by Elizabeth Beresford.
Green Dragons are much more varied & have subsections - Pony Books & Adventure Books - authors include Mary O'Hara the Pullein-Thompsons, PC Wren Anthony Buckeridge & Edgar Rice Burroughs. There is also an early Dr Who book (the Crusaders, in case UQD is interested)a Noel Streatfield & an Arthur C Clarke, amongst others.
They were on sale for 3/- or 2/10d if you bought two or more. (Presumably you would have to send a Postal Order)
"Why not determine to buy a Dragon Book a week? In such a short time you would have a collection to be proud of, gay in colour, bringing brightness to your room!"