Complex issue, improvements (and warning ?) required IMHO. On this CM93 chart(46° 08.5 N 1° 12.5 W) there are (at least) 4 depth contours (isobaths): 2, 3, 5 and 10 m.
Mariners standard, Zoom detail 5, 4 colors checked in Toolbox,
Select Shallow depth 2m, Safety
depth 5m, Deep depth 10m and everything is fine (pic)
Now select Shallow depth 3m: no change, unless you move that part of the chart off screen
, or reopen the toolbox, or ... => refresh problem
Now select Shallow depth 4m: no visible change at all as there is no 4m isobath on that chart.
Select Shallow depth 5 m = Safety depth : you loose one colour (that's fine) but there is also a refresh problem if you revert to Shallow depth 2 ou 3 m.
(see pic of partial refresh)
Shallow, Safety, and Deep depth settings should be set to the isobaths present on the specific chart one is viewing to take advantage of the 4 colour scheme.
In other words, the same shade of blue colour will not code the same depth on different charts
. (see last pic)