A white collie is as healthy as every other collie.
It results from 2 parents having the gene for large
white markings, known as white factoring, which is
inherited independently from the coat color,
(sable, tri and blue).

Aries Irish Angels Destiny

Sable headed white

A white collie has received the white factoring
gene from both parents and it's coat is mainly
white with a fully colored head.
  If a collie only has one gene for white factoring,
it will usually have a white stifle and a "full"
white collar, sometimes even white body spots,
known as a "white factored collie".

CH Special White Christmas

Blue headed white

You need to have 2 WF genes to have a white
collie. These white collies are as healthy as their
colored brothers and sisters. They have, if their
parents are clear of any genetic disorders, the same
hearing and sight as other collies. This white collie
is not to be confused with a double dilute merle
collie.

Dynasty's Vanilla Skies

Sable Merle Headed white

A double dilute merle is the result of the semi
lethal merle gene, which happens when 2 merles
(EX: blue merle to blue merle, sable merle to blue
merle, or sable merle to sable merle) are bred
together. The merle gene acts as a "dilution gene"
which not only affects coat color, but also vision
and hearing.
Many double dilutes are deaf, and or blind,
although there have been several born with
Normal eyes and normal hearing.

Tri Factored Sable headed white