Every other line is white.
Is this correct?
I know we say "I play tennis every other day" to mean I take a day off and then play then a day off then play, but in the example, it is the subject.
So it felt strange?
The sentence, "Every other line is white" is valid.
Note that there are actually two possible interpretations of every other:
Generally, the "alternating" interpretation is correct, but occasionally the speaker means "all others". If it's not clear from the context, you may need to seek clarification whether the speaker means "alternating" or "all others".
That's right. It means one line is white, the next is not, the next to next is again white and so on.
As a native english speaker (UK / Scots) I'd listen to "Every other line is white" and immediately think -- this line is black and all the other lines are white.
Better would probably be "every second line is white".
English is a stupid language.