You are comparing component by component. You are not testing whether any of the RGB components for a pixel are greater than zero: you are testing whether the individual components are greater than 0. When you have pure colors like you have, one or two of the RGB components are 0, and those stay 0 with your code.
pic1 = imread('Testcircls.tif');
obj = uint8(ones(size(pic1)));
mask = repmat(any(pic1 > 0, 3), [1 1 size(pic1,3)]);
pic1(mask) = 255;
obj = pic1;
image(obj)
