They just changed the standard colour of the plug / cable - they are now the same as the Galaxy Note 2's plug/cable.
All Galaxy S2's came with black plugs - All Galaxy S3's came with black plugs and now all Galaxy S4's come with white plugs. There was no matching of colours for any particular reason by Samsung.
As an aside note - I bought the Galaxy S3 in White last year it had Black plug / black cable - white earphones. This year I bought the Galaxy S4 in Black and it has white everything. Very vice versa.