Create a mask:
greenDataIdx = Depth <= 11.9;
plot( Lat( greenDataIdx ), Long( greenDataIdx ), 'g*' )
hold on
...
Then plot your other data similarly. In your example there is no data with depth > 11.9. I was going to ask what happens to data > 19.8, but there isn't any. You can do the red data just the same though anyway, if any exists:
redDataIdx = Depth > 11.9 && Depth <= 19.8
An if statement where the target of the if is a vector is very rarely what you want. It will evaluate to a single scalar for the entire array.
