Is "kasinas" meditation literally envisioning a circle or disc of color
It's a popular legend.
'Kasina' means 'whole', and the aim of kasina practice is to make the mind one-pointed, single-focused on the basis of concentration:
 "There are these ten totality-dimensions (kasina-ayatana). Which ten? One perceives the earth-totality (kasina) above, below, all-around: non-dual, unlimited. One perceives the water-totality... the fire-totality... the wind-totality... the blue-totality... the yellow-totality... the red-totality... the white-totality... the space-totality .. the consciousness-totality above, below, all-around: non-dual, unlimited. These are the ten totalities. Now, of these ten totalities, this is supreme: when one perceives the consciousness-totality above, below, all-around: non-dual, unlimited. And there are beings who are percipient in this way.http://www.accesstoinsight.org/canon/su ... 0-029.html
See more on this term in the thread:http://www.lioncity.net/buddhism/index. ... opic=31453
Discs of color can be optionally used as learning devices for the practice of color kasinas. As described in the Visuddhimagga, one might as well use the flowers of suitable color, or whatever. It's not a 'visualization', but rather attunement of recognition (saññā) to the basis of concentration.
For air kasina, one can use either visual contact with air, or air contact. So the kasina isn't necessarily practiced through visual means. In Anapanasati jhana practice, which is essentially a practice of air kasina (see Dhammasangani-Atthakatha 200), the element of air is attuned to through the tactile contact.
Kiṃ pana pathavīkasiṇaṃ ādiṃ katvā aṭṭhikasaññāpariyosānāvesā rūpāvacarappanā, udāhu aññāpi atthīti? Atthi; ānāpānajjhānañhi kāyagatāsatibhāvanā ca idha na kathitā. Kiñcāpi na kathitā vāyokasiṇe pana gahite ānāpānajjhānaṃ gahitameva
; vaṇṇakasiṇesu ca gahitesu kesādīsu catukkapañcakajjhānavasena uppannā kāyagatāsati, dasasu asubhesu gahitesu dvattiṃsākāre paṭikūlamanasikārajjhānavasena ceva navasivathikāvaṇṇajjhānavasena ca pavattā kāyagatāsati gahitāvāti. Sabbāpi rūpāvacarappanā idha kathitāva hotīti.
"But is this all the absorption belonging to the consciousness of the sphere of refined form, beginning with the earth kasiṇa and ending in the perception of the skeleton? Or is there anything else?"
"Yes, there is. There is ānāpāna jhāna and the development of kāyagatāsati, which have not been spoken of here."
"Because ānāpāna jhāna is included in the air kasiṇa
; the development of kāyagatāsati arisen by virtue of the fourfold and fivefold jhānas with reference to the hair etc., is included in the colour kasiṇas; the kāyagatāsati produced by virtue of the jhānas attending to the unattractiveness in the thirty-two parts of the body, and that of the jhāna attending to the colours of the nine kinds of corpses in the charnel grounds is included in the ten repulsive things. Thus all the absorptions of consciousness connected with the sphere of refined form have been included here."
Best wishes, Dmytro