The particular blue would hard to find  or create though the use of magic did make things easier. He pondered the exact transfigurative spell to use to achieve that.... though knowing this girl if he used magic  on the portrait and made it into a living portrait it would probably be the prickliest, most argumentative portrait that he had ever made.

The change in his train of thought has obviously been detected.

"Wondering what exact shade of blue your eyes are." He replied on impulse. That was safe enough, mentioning that he'd gotten distracted by the lines of her neck was probably less so.