把matlab双坐标轴的颜色设置为黑色

[AX,H1,H2] =plotyy(x,y1,x,y2,@plot);% 获取坐标轴、图像句柄
set(get(AX(1),'ylabel'),'string', 'the utility of bank');
set(get(AX(2),'ylabel'),'string', 'optimal R of borrower');
xlabel('number of iterations');
set(H1,'Linestyle','--');
set(H2,'Linestyle',':');
legend('utility of bank','optimal R of borrower')
set(AX(:),'Ycolor','k') %设定两个Y轴的颜色为黑色