Excel Fill Color Changes without manually changing it

My Excel "Fill Color" changes on actual saved documents, for instance I will
fill a row yellow for categorizing work, and one light green. Then later
and/or after I open it next the yellow and green are the same. Also, when I
go to use the same blue, the next day it's a different blue. This is killing
me, when my leads are categorized I don't know which was yellow, which was
green and if I'm updating the next day I need to fix all the inconsistencies
in my key and chart. Please help if someone knows why. Thanks.