Some Simple Genetics.

The wild Peachface lovebird is Green.  

The Blue mutation is a result of the lack of Yellow colouring. 
Not all of the Yellow is missing however so the resulting bird is a greenish blue.

The Yellow mutation is a result of the lack of Blue colouring.  
There are several different Yellow mutations with varying intensities of Yellow.

If we breed with these Blue and Yellow mutations we can end up with birds which are missing both the Blue AND Yellow colourings resulting in a pale Yellow to Cream to White bird. 

Some Examples:

Blue x Lutino = Cremino.  A pale yellow bird is the result because of the intensity of the Yellow in the Lutino.
Blue x Australian Yellow = Primrose.  A pale cream bird because the Aust. Yellow is not quite as intense.
Blue x Cinnamon = Ivory.  An almost white bird with very pale greenish tinge. 

NOTE that the Whiteface Blue is a much more pure blue. This should result in paler colours in the crossings above.

NOTE ALSO that an understanding of recessive and sex linked genes is needed when breeding Peachfaces for these colour mutations as Lutino and Cinnamon are sex linked.  Fischers and Black masked colours are only recessive.

Dark Factors:
Birds with single or double dark factor have intensified colours. 

Pied:
Loss of the blue colouring in patches which gives a green/blue bird patches of yellow/cream. Birds can be single or double factor pieds and the amount of yellow/cream can vary to the extent that the bird can appear to be yellow with patches of darker colour.