Is it common for economics journals to publish figures in color online and black and white in print? If so, would it be best to submit the color version? And, in the color version, should I differentiate lines in ways other than color (making the color differentiation helpful but redundant) so that the differentiation is still clear if the reviewer prints the article in black and white?
Read the author guidelines for the journal you intend to submit to. Within any field there is variety.
But always assume the paper will be read in B&W, and make it fall back gracefully. This may be by the reviewer, or it may be by a reader who prefers to work on paper and has easier access to B&W printing. This also goes quite a long way towards making it accessible for colour-blind readers.
"Helpful but redundant" is a good description of what the colours should be. Note that if there is a print version with monochrome figures, the journal will probably require that it's useful n that format; I've never come across journals that will accept two versions of the same figure (but I'm in a different field)