The ruler color can be set using 'ycolor'. When you use legend(), explicitly give it a vector of object handles as the first argument.
For both these tasks, you're going to have to keep track of the objects you're creating. You can use getaddaxisdata() to get axes handles, or you can use findobj(), etc. All plot (line) objects are in the first axes that was created with the first plot() command. The other axes objects only serve to host extra ruler objects.
% fake data
x = 1:10;
y1 = rand(1,10);
y2 = rand(1,10);
y3 = rand(1,10);
hold on
% Plot the first set of data using the main y-axis
plot(x, y1,'r', 'LineWidth', 1.0)
% Plot the second set of data using the second y-axis
addaxis(x, y2,'b', 'LineWidth', 1.0)
% Plot the third set of data using the third y-axis
addaxis(x, y3, 'k','LineWidth', 1.0)
% add a constantline or two or three
line = xline(5,'--k','LineWidth',1.0);
% set axis labels
addaxislabel(2, 'Zdr (dB)','FontSize',12)
addaxislabel(3, 'Rhohv','FontSize',13)
% get axes handles
hax = getaddaxisdata(gca,'axisdata');
hax = [hax{:}];
hax = [gca hax(1,:)];
% only get line objects in the base axes
% flip the handle vector, since they're in order of ascending age
h = flipud(findobj(hax(1),'type','line'));
legend(h,{'Z','Zdr','Rhohv'}, 'Location','northeast','FontSize',7)
% set the color of the yruler for the base axes
hax(1).YColor = h(1).Color;
hax(1).FontSize = hax(2).FontSize;
grid on
See also:
