I've been changing strings a lot lately just to see what feels better and i noticed that earnie ball slinkys have a silver-colored core and d'addario hybrids have a brass colored core. Do some electric strings have a ferris core and some not? Are some strings better for producing more volume through the pickup?
All electric strings that I know of have a ferrous core - they probably wouldn't work very well otherwise.
There are lots of different metallurgical combinations used in the core and wrap of strings, for all sorts of reasons. They're not all the same color, as you've seen.
As you suspect, some of these alloys have stronger magnetic properties than others, which can result in minor output differences.
I always assumed that core string was the sound making string and the wrap was a non-ferris wire used to tune the vibration. Hence nickel wound steel. The color differences made me think the opposite.
Last edited by geo-rage at Aug 23, 2016,
Electric guitar strings have ferromagnetic windings that interact strongly with the magnetic fields - nickel, nickel-plated steel, stainless steel etc. OTOH, acoustic strings mostly use non-ferromagnetic windings chosen for good acoustic properties. The cores are the same though.