Try this:
sprintf('%.8f\t', whitepoint('d50'))

This will return more than 4 significant digits.

I'd suggest you increase the precision.  However, calculating the values using the formula Carnë mentioned is not trivial.

I am reviewing my copy of CIE 15:2004, which I think will have a good suggestion for calculating the whitepoints.  I'll post more here later.

Of course you can. Looking forward to the fixed version.

Hi!

Aside the case insensitivity issue, which can be solved with using

  switch (tolower (string))

there's a few other issues:

1) no documentation. Could you please write documentation in TexInfo. See other functions in Octave for an example. It's pretty simple.
2) your function only has 4 digit precision, but there's an actual formula which can give a much correct value. See http://en.wikipedia.org/wiki/Standard_illuminant
3) no input check. For this case, the function should start with:

function xyz = whitepoint (standard = "icc")
  if (nargin > 1)
    print_usage ();
  elseif (! ischar (standard))
    error ("whitepoint: STANDARD must be a string.");
  endif

Note on how we are also setting the default value right at the start.

In addition, other things that are just the guidelines for contributions

4) the main function block should also be indented
5) use double quote for strings unless (there's a really good reason otherwise)
6) error message must mention the function name, the variable name (in caps) as it's named in the documentation (you don't have documentation yet so I'm calling it standard), and the wrong value that was given. So it should be something like:

  error ("whitepoint: unknown STANDARD `%s'.", standard);

Note that if you made input check as I mentioned above, you'll know it's a string, so you can use %s.

7) specify what block is ending with endfunction, endswitch, etc...

XYZ color values of standard illuminants

usage:
xyz = whitepoint(string)
string is one of the following:
a, c, icc, d50, d55, d65'