I shall attempt to explain how the dark factor works without going into any complicated reasons
All budgies have a dark factor rating. It is either 0, 1 or 2. This affects the base colour regardless of what mutation the birds are
0 dark factors appear as sky blue or light green
1 dark factor appears as cobalt or dark green
2 dark factors appear as mauve or olive
If you pair two birds with 0 dark factors together the chicks will also have 0 dark factors. It doesn’t matter whether they are both sky blues or both light greens or one of each
If two birds with 1 dark factor are paired together the result will be 50% 1 dark factor, 25% 0 dark factor and 25% 2 dark factors
If two birds with 2 dark factors are paired together all the chicks will also have 2 dark factors
If a 0 dark factor bird is mated to a 1 dark factor bird the result will be 50% with 0 dark factors, 50% with 1 dark factor
If a 1 dark factor bird is mated to a 2 dark factor bird the result will be 50% 1 dark factor, 50% 2 dark factors
If a 0 dark factor bird is mated to a 2 dark factor bird the result will be 100% 1 dark factor chicks
Mutations like albinos, lutinos, greys and others where the dark factor rating may not be visible still have a dark factor rating but it might be only possible to tell what it is by breeding.
e.g. If a mauve cock bird (2 dark factors) was mated to a lutino hen and the chicks were all dark greens and/or cobalts (1 dark factor) it would prove that the lutino’s dark factor rating was 0. If however the chicks were a mixture of cobalts, mauves, dark greens and olives, the lutinos rating would be 1 dark factor. If they were all mauves & olives her rating would be 2 dark factors.
Mating a bird with 2 dark factors with a bird with any other rating can not possibly produce chicks with 0 dark factors. Also mating a bird with 0 dark factors with a bird with any other rating can not produce chicks with 2 dark factors. This means that a sky blue cannot have a mauve parent and an olive can not have a light green parent