Re: why does milk show as a base on red litmus paper and an acid on wri paper?

I am not familiar with wri paper but I think I can still explain what you 
are seeing.  Milk is a very complex product containing, among other things, 
some weak acids.  You said you measured the pH at 6.5 (the range is 
typically 6.5 to 6.8), which is very close to neutral.  The use of litmus 
or other papers is a good way to check what general pH range the product is 
in, rather than an exact value.  Remember that there will be uncertainty in 
any measurement you take and you should always be aware of what that 
uncertainty range will be.  I would recommend that you investigate the 
litmus and wri paper ranges of uncertainty to see where your expected end 
value of 6.5 falls.

Second, two different test results (like you saw here) usually indicate a 
true value that lies between the two results recorded.  Milk is close 
enough to a neutral pH that the different results really aren't very 
surprising.  The age and condition of the litmus and wri paper could also 
affect the results of the test (something else to take a look at).  But 
basically, milk is a very weak acid, so the wri paper is giving you the 
result I would expect.  If you have other methods available to you to test 
the pH (like a pH meter or titration), I would probably rely more heavily 
on a more accurate test method than just the papers.

I hope this information helps you understand your conflicting results.  
Milk is a difficult material to know well.

Kieran