There are two main tasks the colouring of the plumage should fulfil: attracting attention and camouflaging.

Obviously, the priority for the Lineolated Parakeet is camouflaging. In the green foliage you can hardly distinguish the plumage from its surrounding. The black plumage colouring imitates perfectly the play of light and shadow in the tree tops.
Now it becomes clear, that a courtship with the presentation of a colourful plumage, long tail feathers or a lovely crest do not fit at all to the Lineolated Parakeet. He "deals with it" just by the way and inconspicuously.
Nevertheless, the plumage of the Lineolated Parakeet is not boring or monotonous. As already discussed in the description of the Parakeet, the feathers show varying shades of colour.

Now, I want to deal with the feathers in particular, as well in view of the colouring, which is important for the genetics (see:

Lineolated Parakeets belong to the nidicolous birds. They hatch more or less naked and are still blind for a few days. Firstly, the down feathers, which are an independent feather generation, are white. They solely act as a regulation of warmth. Later they are replaced by the youth plumage. This shows grey downs, which also can be observed at the adult birds. Soon the first contour feathers begin to sprout, primarily at the tail and wings, later at the whole body.
Not until the first moulting period, the youth moult, one can speak of the adult plumage, even if you can hardly distinguish between the young and adult birds among the Lineolated Parakeets.

As among all birds there are different kinds of feathers which can be divided into different groups depending on their shape and/or function. The following listing should give a little overview:

Once a year an ever returning phenomenon can be observed at the birds - the moult. This is a hormonal caused process in which great parts of the plumage will be more or less exchanged or renewed. At the Lineolated Parakeet the moulting period takes place in autumn (September/October/November) as a partial moult. This means, that not all wing feathers will be dropped at the same time, so the bird remains able to fly. On account of growth and hormonal rearrangements, especially younger birds can moult several times a year. The so-called youth moult, this is the first moulting after leaving the nest, can be quite intense. Than the bird seems to be ruffled and incomplete in a way.
As you can imagine, this process of renewing plumage is strenuous for the bird. The organism of the bird is not that resistant anymore and there is an increased risk of infection. Often the birds are more quiet than and especially dependent on a rich, well-balanced nutrition.

The colours of the feathers are a complex interplay of pigment colours and feather structures. Especially the melanins and lipochromes are known as colour pigments. On account of their molecular structure they absorb determined wavelengths of the white light and so they appear in different colours. For example, the eumelanin is more dark-brown to black, in contrast the phaeomelanin seems to show a reddish-brown or yellow tone. Furthermore, there is a trichosiderine-related melanin (red-brown to crimson) and the erythromelanin (chestnut brown).These melanin particles will be stored in the feathers and give them among other things mechanic stability.
Probably existing plumage marking emerges by the release of melanin through rhythmically appearing metabolism products which will be either inhibited or induced increased. As the lipochromes belong to the carotinoids they are fat-soluble. They are taken in with the help of fatty feed. They are responsible for yellow colours which especially can be found in the outer layers of the feathers.

The feather colours white and blue as the iridescence depend on fine structures of the feathers as well. White is based on the physical process of refraction and reflection of light. The light hits lots of pigment-free and transparent horn cells of the radius of feather and will be scattered and broken in all possible colours there. The sum of these colours is white. However, this plumage colour has to be distinguished from the albinos which are missing every single dark pigments due to an error in the melanin synthesis, whereas the "normal-white" birds absolutely have pigments.
In the same way you cannot attribute the blue colour of the feathers to a blue pigment, but rather to the existence of a so-called cloudy layer. These are cells filled with air which are lying above the loaded with dark colour pigments cells of the feather branches. Because of that the light is not reflected white totally, but just blue. If a yellow pigment is stored in the cortical layers lying baove the cloudy layer additionally, by existence of pigments in the lower layers this feather seems to be green, otherwise yellow.

Now, with this knowledge it is easy to explain how the different
colours of the Lineolated Parakeet emerge. Two different genes, transmitted independently from each other, determine the colouring of the plumage, same as at the budgie (Melopsicattus undulatus). One gene is responsible for the pigmentation (lipochrome) of the outer parts of the feather, a second for the pigmentation (melanin) of the feather core. The gene for the production of the melanin is placed on the sex chromosomes (heterosomes) of the birds, more precisely said on the X- or rather the Z-chromosome, of which the male bird has two and the female bird just one (also see: explanation of technical terms). Nevertheless, this special gene does not generally need to be responsible for the "production" of the melanin. It already would be enough, if e.g. a preliminary product of the pigment could not be produced or that a small step in between does not work at all. The result would be the same: The organism cannot produce any melanin. In contrast, the other gene is placed on one of the autosomes. As lipochrome is taken in with food, it cannot lead to any problems concerning the production. Nevertheless, other complications could appear. Imagine, that the lipochrome would be taken in, but would not be transported further anymore. Then, it would just lay about and at least possibly would be degraded. This is just one possible explanation for the absence of the yellow feather pigment.
First at all, let's have a look at the normal green Lineolated Parakeet. All metabolic pathways for the production of the melanins and for the storage of the lipochromes run off impeccable. In the feather core there are black melanin particles, the cortical layer contains a yellow lipochrome. The bird appears green. If e.g. the further transport of the lipochrome will be stopped, on account of a mutation on a chromosome, this pigment cannot reach its actual "destination". The outer cortical layer remains colourless. The above explained phenomenon of the light refraction lead to a blue-looking bird. As already described at the mutations (
mutation/turquoise), the integration of a yellow lipochrome cannot be prevented completely at the Lineolated Parakeet. Therefore, a "perfectly" blue bird does not exist. There always is a light yellow gleam. Therefore the "blue" Lineolated Parakeet preferably is named "turquoise".
Now, let us assume that all lipochromes reach the outer feather layer. By another mutation, this time concerning the gene for the production of melanin, it is prevented that melanin will be assembled, further transported or placed in storage. The result would be a yellow bird. But not only that! Because the production of melanin is stopped completely, the bird additionally has got red eyes and light claws. So, we have a lutino.
If, at least, these two mutations meet, the plumage of the arising "albino" is more or less pigment-free (except from the already above-mentioned yellow gleam, so these birds are called "Creaminos").