I have had my guitar for about 7 months or so now (this is my first) and i just learned a few weeks ago that you are supposed to change the strings every so often. well a couple of weeks before that i noticed that my two higher, unwound strings sounded more dull than usual. The wound strings were coated when I bought the guitar, but so far, they still seem to sound fine. Is it possible for the unwound strings to wear faster than the wound strings? Or is there something else that may be causing the difference in sound I am hearing? The thing that gets me the most, it it seemed almost like it was overnight, no one else in my family really notices it, but i can definitely tell. I just find it strange that it was only the unwound strings that were affected, but still both of them sound different. Either way I am planning on putting some new strings on soon and I will be replacing them as a set, but I am just wondering what could be causing this.
What you are probably feeling and seeing is those unwound strings rusting. They look dull, even black, and sound dull as well - and they feel rough when sliding your finger along them.
Wound strings do rust but the effect takes longer to be noticeable, partly as the strings are thicker, partly due to the different wire wound round the core.
I would still recommend replacing the whole set just to brighten up the tone and avoid breakage.