Thanks so much for your explanation.
Indeed, chick no 1, the blue and white one, has a very diluted type of blue on the lower tummy but then, on the back where the wing tips intersect, the blue is even brighter. I would explain the blue in terms of mixing poster colours - I would mix blue into white to get light blue and then put in a dab of black to get it greyer.
Chick no. 2, the yellow/green one, its green is almost a very light shade of olive. It looks as if its a normal green budgie colour which has been smeared with a hint of grey over it.
The three chicks of this clutch have their colours looking as if they were in an old colour photography, you know, with those diluted and rather pastel/shady colours.
I can see the difference of the mutation when comparing them with Dusky, a female new addition this year. Her blue is the 'Dodger Blue 2' on her chest - the link for the colour chart I'm trying to match with is this - https://www.ljmu.ac.uk/cwis/colors/Blue.htm
Then she has very dark and dramatic black markings starting from heard head and going down to cover, entirely, her wings. Then she has the purest white of white's face. Really distinct colours.
Whereas all the others, from my own breeding strain, are all pastel like in colour except for the bright green of PapaBudgie, the father.
Thanks for your encouragement re. the degree. You are right, English opens up many possibilities and its a wonderful subject and also a passion of mine.