[Grammar] in manual/automatic/ mode

[kadioguy]

In Macmillan English Dictionary, it says:

mode n.

[countable] one of a series of ways that a machine can be made to work:
in manual/automatic mode
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If the word 'mode' is countable, why don't it place any determiner or article? for example: in a manual/automatic mode.
＝＝＝＝＝＝＝＝＝
Also in LDOCE6, it says:

mode noun [countable]

technical a particular way in which a machine or piece of equipment can operate:
Set the monitor to 256 colour mode.
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If the word 'mode' is countable, why don't it say 'Set the monitor to a 256 colour mode'?

Thanks!

PS I also posted the same question in this, but all of your answers are unique to me. Thank you.

 

[Raymott]

There is only one 256 colour mode. So you don't get a choice of [256 colour] modes.

 

[kadioguy]

Why not use 'the' 256 colour mode and 'the' manual/automatic mode?

 

[Raymott]

You could write that, but it's not necessary. "256 colour mode" acts a descriptor. Here are some other examples:
"You can have strawberry or chocolate ice cream". "I'll have [the] chocolate, please."
"They taught both Hindu and Buddhist styles of meditation. I chose [the] Buddhist style."

In the latter, "Buddhist style" refers to "Buddhist style meditation".
In your example, "256 colour mode" refers to "256 colour mode graphical display" (or some other technical term). So, while 'mode' and style' are nouns, they act here like compound adjectives and the real noun is left out.
That's how I see it.