load V.mat
load U.mat
load JPDF.mat
Add an extra column of appropriate values to U, V, and JPDF:
U(:,end+1) = 2.5;
V(:,end+1) = V(:,end);
JPDF(:,end+1) = min(JPDF(:));
figure(); % separate figures for demonstration
contourf(U,V,JPDF,12,'edgecolor','none')
colorbar
caxis([0 0.3]);
hold on
plot([0 0],[-3 3],'k--',[-3 3],[0 0],'k--')
xlabel('$u`/\sqrt{\overline{u`^2}}$','interpreter','latex','FontSize',12)
ylabel('$v`/\sqrt{\overline{v`^2}}$','interpreter','latex','FontSize',12)
ax = gca; % current axes
ax.FontSize = 15;
hold off
axis([-2.5 2.5 -2.5 2.5])
Alternatively, you can achieve the same effect by setting the color of the axes to be the first color in the colormap (i.e., the color of the minimum value of the contour):
figure(); % separate figures for demonstration
load V.mat % restore original data
load U.mat
load JPDF.mat
contourf(U,V,JPDF,12,'edgecolor','none')
colorbar
caxis([0 0.3]);
hold on
plot([0 0],[-3 3],'k--',[-3 3],[0 0],'k--')
xlabel('$u`/\sqrt{\overline{u`^2}}$','interpreter','latex','FontSize',12)
ylabel('$v`/\sqrt{\overline{v`^2}}$','interpreter','latex','FontSize',12)
ax = gca; % current axes
ax.FontSize = 15;
hold off
axis([-2.5 2.5 -2.5 2.5])
cmap = get(ax,'Colormap');
set(ax,'Color',cmap(1,:));
