Typically a Canvas can hold a specific range of colors which is
referred to as the depth of the
canvas. By default the number of colors a Canvas can hold is
'millions' which gives essentially photo-realistic images.
The depth of a Canvas can be reduced. You can change it to
'thousands' or '256' or '16' or '2'. If you do this you will
generally lose quality but it will reduce the amount of memory used
and can speed up operations.
However you should avoid reducing the depth of a Canvas. Some
operations do not work properly unless the depth is 'millions' and
others may not work quite as you expect due to the color
compromises that have to be made.