MATLAB axes colors change when writing a GIF - matlab

Question

Relevant Information: MATLAB R2015b, Mac
I am currently trying to write a GIF from a series of datasets (txt files) I took. I have no trouble writing the GIF file, however, when played back (in PowerPoint) or previewed in the OS X Finder, the axes labels change in color. In addition to the color change, I receive this warning:
Warning: Image data contains values that are out of range. Out of range values will be given the nearest valid value.
Currently, I grab all the data files in the directory, plot them, grab each frame, and put them into a GIF. Here is my code:
%Create MATLAB movie from plots
clearvars
%addpath('C:\Users\ucmuser\Documents\MATLAB')
filename='cooldown_movie.gif';
ext_in='txt';
[~,listing]=f_search(ext_in);
[r,~]=size(listing);
listing=natsortfiles(listing);
F=figure;
%r=20 is used for quick debugging (original is 460 files).
r=20;
% y=linspace(1,2*pi,100);
% x=linspace(1,100,100);
%C(1,1,1,r)=0;
for i=1:r
A=dlmread(listing{i});
listing{i}=strrep(listing{i},'_','\_');
x=A(1,:); %X Array
y=A(2,:); %Y Array
plot(x./1E9,y.*1E3,'-','LineWidth',1.2,...
'Color',[0.8500 0.3250 0.0980]);
grid off
xlabel('Frequency (GHz)','FontSize',18,'FontWeight','bold')
ylabel('Voltage (mV)','FontSize',18,'FontWeight','bold')
title('Cooldown Movie','FontSize',24,'FontWeight','bold')
G(i)=getframe(gcf);
drawnow
frame = G(i);
% im = frame2im(frame);
[C(:,:,1,i),map] = rgb2ind(frame.cdata,256,'nodither');
% if r == 1;
% imwrite(C,map,filename,'gif','LoopCount',Inf,'DelayTime',0);
% else
% imwrite(C,map,filename,'gif','WriteMode','append','DelayTime',0);
% end
end
imwrite(C,map,filename,'gif','LoopCount',Inf,'DelayTime',0);
A sample image is shown below. The axes labels change in color. If I turn on the grid, the effect is even more apparent with the grid changing in grayscale intensity. I've tried setting limits and the effect is still present.

Answer 1

I've found a solution for the time being although I'm not quite sure why it works. I started going through the figure properties and turned off the GraphicsSmoothing property (type F=figure;F.GraphicsSmoothing='off'; before the "for loop"). This seemed to fix the problem as well as clear the original out of range error.
With the GraphicsSmoothing property turned "on" and going through the first 20 data files, I have a maximum indexed color value of 22 via max(max(max(C))). Performing the same code with the GraphicsSmoothing turned "off" yields a maximum value of 4.
I am attaching two images displaying the extreme color differences with the GraphicsSMoothing turned on and off.
GraphicsSmoothing turned On (default)
GraphicsSmoothing turned Off
If someone knew why this is the case, I would greatly appreciate it.
UPDATE: I attempted to use this solution with grid on and the background border turned orange. I am confused on this whole issue.