Hi l want to paint over some section of the interior of my house, theres about three differnet shades of white the doors , trimms and ceiling, l dont know the correct names of what shade of white each one is, how do l find out what is the correct shade of white to buy. l tried those paper color matching things you have in the paint section but l never seem to get or find the correct matching one , the only way l can tell for sure is to actually buy a paint sample to see if its a match and if its not then l have wasted my money