Will take a long time learning by experience . .
This may help . . .
STEP #1 . . YELLOW cock X FAWN hen . . . OFFSPRING: All will be normal in colour (PROVIDED THERE ARE NO HIDDEN "SPLITS" IN THESE PARENTS)but COCKS will be split for both FAWN and YELLOW; Hens will be only split
for yellow and will be of no interest to us
STEP #2 . . DOUBLE SPLIT cock X YELLOW hen . . . OFFSPRING: There are many possible results but among them the ones that are of interest are
YELLOW-FAWN hens (You Beaut!!

) and YELLOW cocks that are SPLIT for FAWN (unfortunately these cannot be distinguished from the yellow cocks that are NOT split for fawn
STEP #3 . . This is a trial and error step if you are using your YELLOW possible split for FAWN cock . . . but mate him with a YELLOW-FAWN hen . . . If any YELLOW-FAWN cocks (You Beaut!!

) appear then you know that he is indeed split for fawn
Obviously you can purchase a YELLOW-FAWN cock to eliminate the trial and error of step 3 . . . but step 3 is the way you would have to go if you were actually producing a brand-new "combination" mutation
PS A key word in all of this is OUTCROSSING . . . incorporating healthy unrelated birds from time to time into your programme
Simple really!?!?! . . . Well not quite so simple as because, since the hoped for offspring will not occur in every nest, periods of good fortune may be few and far between as Mr Murphy applies his laws to those of Br Mendel
PPS The above steps apply when combining any SEX-LINKED mutation with any AUTOSOMAL-RECESSIVE mutation . . . whether the birds be finches, parrots, quails . . . or Strutting Ducks