It will be good to go over this issue during your
MRI introduction in the classroom. This question
may be asked by your students to you as well.
Please see the sample Chickscope questions and
answers in the tutorial.
Actually, the MR image has no "colors" at
all - it is a radio signal. But in order to
make it visible, the computer artificially
colors it. Since a single image conveys only
one value at each point, we choose to assign
that value to "brightness".
It could also be assigned to a color,
however, to make a "pseudo-color image". If
you have the right software in your
computer, you can change the colors to suit
The black and white choice has other
bases, too. MRI is often done along with
X-rays in a hospital radiology department.
The doctors are used to looking at black and
white x-rays, so it was natural to do MRI in
the same way.