There has been many problems associated with this mutation. 
The most trying of all was trying to locate one. 
This particular mutation of gouldian has four names it can be called 
depending on where you live. 

-dilute backed ( Canada parts of europe)
-suffusion ( netherlands) 
-pastel ( united states, some parts of S America)
-light back ( Canada, small Island Nations)
 
This regional name problem has caused many breeder's great difficulty just trying to
locate a  pair in a country or province/state other than their own.
The newest name "pastel green back" was suggested by the American organization the 
National Finch and Softbill Assoc. They suggested this be it's "universal" name. 
Unfortunately, many countries breeders are still sticking with their own choices. 
In many countries the name pastel just does not seem to fit this bird. 
The other confusion is pastel is what a Df Bb/Yb mutation is known as. 
So to this date no "universal" name exists.  
As I am in Canada I will stick with the name we best know and is 
most widely used for this breed: Dilute Backed. 
Perhaps this should be it's universal designation as it suits the mutation, simply because 
this mutation is a dilution of colour as you will see in the rest of this article.  
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Rob Marshall goulds, finch, finches,  The yellow factor in gouldians is a sex linked trait. 
This means males from yellow hens will be yellow and hens from yellow fathers will be yellow. 
Yellow backed birds start off with red eyes that go brownish black or black
shortly after feather sheaths start to open. 
They are easily distinguished from the normal or green backed bird as they are brownish or yellowish instead of dark gray
days after they are hatched.
All gouldian chicks start off pinkish but darken as their quills develop. 
Dilute Backs (Dbs) are usually a tawny or jaundice yellow at this age. 

There are a few different known mutations of the yellow factor gene.
They are: 

Single Factor Dilute Backed (sfDb): These gouldians are always purple breasted( and 95% are always male) 
The colouration is very similar to the green backs; however single factor gouldians cannot produce melanin so the black bib 
looks creamy grayish blue. 
 
Single Factor white breasted Dilute Backed (sfDb wb): These gouldians are genetically similar to the purple breasted mutation; 
however the white of the breast can dilute the bird so that it is a lemony instead of lime yellow.
The difference between them and double factor gouldians is only apparent in the bib and collar area. 
Single Factor pastel Green back white breasted gouldians have a greater lack of expressing melanin and so 
their bibs are grayish, instead of the bluish white hue of double factor yellow backs. 




Double Factor Yellow Backed (Df Yb): Yellow backed purple breasted gouldians unlike the single backed gouldians cannot express melanin at all 
and as a result the bib has a very light bluish tinge. The collar is much lighter then sfgb wb gouldians, and is slightly darker 
bluish white then the bib. This characteristic makes them easily distinguished from sfDb wb gouldians because of their apparent grayish bib and 
darker less suffused collar.


In the white breasted DF Yb the bluish, and black areas are replaced by chalk white.
Also because of the white breast all green in the pigment is also lost giving the wb DF Yb the clearest canary yellow 
plumage so desired in this sex linked mutation.  
This means that when viewed because of the lack of contrast of the bib, collar, and chest this bird looks very striking.
The Yellow and white contrast quite well making the bird look very tight feathered and sharp. 
In the black head Wb Df YBs the gouldian looks very dual chromatic ( only expressing white and yellow).
These are sometimes known a "faux" lutinos when black headed as they closely resemble lutinos but have dark eyes. 

    compare with sfDb White Breast below  



To illustrate the dilution we will look at the back pigmentation.
As the bird loses melanin it also loses the structural blue pigmentation as more lutein is expressed.
Eventually what you wind up with is a near total subtraction of structural blue
 in plumage. 

Normal	Single Factor	Single Factor white breasted	Double Factor Yellow back Purple Breasted	Double Factor Yellow back White Breasted

 
We can also compare the collars and note the dilution and eventual extraction of all blue in this as well as the back. 

Normal	Single Factor	Single Factor white breasted	Double Factor Yellow back Purple Breasted	Double Factor Yellow back White Breasted

Finally we can compare bibs and also note the extraction of eumelanin ( black pigment) in gouldian finches as the lutein increases. 

Normal	Single Factor	Single Factor white breasted	Double Factor Yellow back Purple Breasted	Double Factor Yellow back White Breasted

Lutinos: are a very specialized yellow back and are not classed with the other yellow backs which are dilutions.  
This means that the pigmentation is affected by a dilution of the pigment cells.  
However; lutinos are not affected in this way.  They are affected through albinism. 
Albinistic mutations correlate with mutant alleles of loci involved in the process of melanin production including 
metabolism and the construction of melanosomes. The hallmark of an albinistic mutation, compared to sf ybs and df ybs, 
is the reduction in melanin within soft tissue and not just feathers. In other words, they lose melanin 
from eyes, skin, legs, beak and toenails. They show signs of reduced eye colour upon hatching, but unlike the dilutions these 
pigments do not correct themselves.  Melanin granules produced by these colour morphs are altered 
in structure, the change varies depending upon the function of the locus involved, but all 
show qualitative melanin changes.  

  

Mating Chart: Please use the quick grid to locate a 
crossing chart or scroll down to view all the charts. 

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