sanyo spc-3100 battery

the wierdest thing happend when i bought my cell phone i didnt think much of it but i changed the colour of my battery i haad alot of different colours for it but i decided to keep it at blue and i havent bothered to do it again till today it was my brother who remindered me he said he had changed it before so i thought nothing of it untill he said that after about 4 days he couldnt find any sign of ever having the option to do that so i tried it and i cant find it any where has this happend to any one else? or does someone know how to do it? cause ive asked the company and no one knows how to do it but it can be done i know it