If 'GREEN' can be coded as 21, 54, 15, 15, 42 then 'YELLOW' will be coded as :

75, 15, 36, 36, 45, 69

In alphabetical order

GREEN

G is 7th letter 7x3 = 21 ; R is 18th letter 18 x 3= 54 ; E is 5th letter 5 x 3 = 15 ;

E is 5th letter 5 x 3 = 15 ; N is 14th letter 14 x 3 = 42.

so it codes 21,54,15,15,42

YELLOW

Y is 25th letter 25 x 3 = 75 ; E is 5th letter 5 x 3 = 15 ; L is 12th letter 12 x 3 = 36 ;

L is 12th letter 12 x 3 = 36; O is 15th letter 15 x 3 = 45; W is 23rd letter 23 x 3 = 69

So it codes 75,15,36,36,45,69