Are the formless jhanas without any consciouness of form at all? I don't understand the formless jhanas.

Does this meditator completely lose consciousness of form, or are they meditating on something formless and, apart from that, as before?

I've read the abhidharmakosa bhashyam, of Vasubhadhu, and he says that a mediator of this sort will be later reborn into a realm without form, so that suggests that there are sentient beings without any of the sense consciousnesses. But I just find the idea impossible, to be honest.

Interesting, one of those books says that some Buddhists claim that there is a residue of visual consciousnesses there.

Can anyone describe to me what the formless jhana is like? I'm especially intrigued as to whether there is any sense of shade to it: if it seems darker or lighter than any given colour I've experienced.
Formless meditations exist. The eight attainments (atthasamapatti) in meditation (attaining Adhicitta) broadly has two parts - namely, the first four rupa jhanas and the four arupa jhanas. These last four are higher stages of mental concentration – the formless states - enable one to gain super-normal powers.

Even though we see the mind as normally inter-dependent with body, there are levels of existence where only mental phenomena exist, with nothing whatever of rupa.  The four 'formless' (arupa) meditative states are attainable from the fourth jhana onwards:

The sphere of infinite space (Akasanañcayatana);
This is attained by transcending any cognition of rupa, by abandoning the metal image that was previously the object of concentration, and seeing that space is infinite.

The sphere of infinite consciousness (Viññaaañcayatana);
In this second state, the focus is on the consciousness that had been aware of infinite space.

The sphere of nothingness (Akiñcaññayatana);
In the third, this object (Viññaaañcayatana) is dropped, and the focus is on the apparent nothingness remaining.

The sphere of neither-cognition-nor-non-cognition (Nevasaññanâsaññayatana).
In the fourth, this object (Akiñcaññayatana) is dropped and the mind is in an attenuated state where it is hardly functioning.

SD 24.11 _ Pathama Jhana Pañha Sutta or (Savitakka) Pathama Jhana Sutta. Progressing in the 1st dhyana.
SD24.12a _ Dutiya Jhana Pañha Sutta or (Avitakka) Dutiya Jhana Sutta. Progressing in 2nd dhyana.
SD 24.13 _ Tatiya Jhana Pañha Sutta or (Sukhena) Tatiya Jhana Sutta. Progressing in the 3rd dhyana.
SD 24.14 _ Catuttha Jhana Pañha Sutta or (Upekkhaka) Catuttha Jhana Sutta. How to progress in the 4th dhyana.
SD 24.15 _ Akasanañcayatana Pañha Sutta. Progressing in the sphere of infinite space.  

SD 24.16 _ Viññaaañcayatana Pañha Sutta. Progressing in the sphere of infinite consciousness.
SD 24.17 _ Akiñcaññayatana Pañha Sutta. Progressing in the sphere of nothingness.
SD 24.18 _ Nevasaññanâsaññayatana Pañha Sutta. Progressing in the sphere of neither-perception-nor-non-perception.