If you want to plot the axes themselves in different colors, here's an example that just works. You can copy and paste the code into Matlab and tinker. What I figured out is that you need to get the gca object and modify the relevant parameters.
XX = 1:10;
YY1 = rand(10,1);
YY2 = rand(10,1);
subplot(3,3,6); % I'm doing a figure with subplot, but "figure" would work too
yyaxis left;
plot(XX,YY1,'k');
ax = gca;
ax.YAxis(1).Color = 'k';
yyaxis right;
plot(XX,YY2,'r');
ax.YAxis(2).Color = 'r';
And in case you need a reminder of the basic color coding:
b blue . point - solid
g green o circle : dotted
r red x x-mark -. dashdot
c cyan + plus -- dashed
m magenta * star (none) no line
y yellow s square
k black d diamond
w white v triangle (down)
^ triangle (up)
< triangle (left)
> triangle (right)
p pentagram
h hexagram
This took a while to figure out. I came across the following unanswered stackoverflow.com post (https://stackoverflow.com/questions/45758444/change-color-of-the-second-y-axis-in-the-matlab-using-new-yyaxis-tool) that helped.