Assuming you did something similar to the code below, you can access the axis properties directly and change the color to whatever you want.
% Create a figure and axis with 2 Y axes
figure;
ax = axes; % or ax = gca();
yyaxis('left');
yyaxis('right');
% Set the color of each axis to black
ax.YAxis(1).Color = [0 0 0];
ax.YAxis(2).Color = [0 0 0];
