The patch command's fourth argument, C, is the color of the that patch. Your C matrix is C=0. That is the problem. Instead use a function that defines the color matrix C. Matlab will scale C matrix to the default color values.
Try this:
% Euclidean distance in 3d
C = sqrt(X.^2 + Y.^2 + Z.^2);
Here is what it does to your code:
%%first download these two files:
%www.mathworks.co.uk/matlabcentral/fileexchange/30923-fast-stl-import-function
%www.cs.technion.ac.il/~gershon/EscherForReal/PenroseTriangleStl1.zip
myStr = 'PenroseTriangle.stl';
[vertices, norms] = import_stl_fast(myStr,2);
n = length(vertices)/3;
X = reshape(vertices(:,1),3,n);
Y = reshape(vertices(:,2),3,n);
Z = reshape(vertices(:,3),3,n);
C = sqrt(X.^2 + Y.^2 + Z.^2);
%%Render
figure;
set(gcf, 'Color', 'white');
as = 0.95;
strt=1;
stop=6000;
patch(X(:,strt:stop),Y(:,strt:stop),Z(:,strt:stop),C(:,strt:stop),...
'FaceColor', 'interp', ...
'EdgeColor', 'none', ...
'FaceLighting', 'phong', ...
'AmbientStrength', as);
strt=6001;
stop=6299;
patch(X(:,strt:stop),Y(:,strt:stop),Z(:,strt:stop),C(:,strt:stop),...
'FaceColor', 'interp', ...
'EdgeColor', 'none', ...
'FaceLighting', 'phong', ...
'AmbientStrength', as);
strt=6300;
stop=6719;
patch(X(:,strt:stop),Y(:,strt:stop),Z(:,strt:stop),C(:,strt:stop),...
'FaceColor', 'interp', ...
'EdgeColor', 'none', ...
'FaceLighting', 'phong', ...
'AmbientStrength', as);
view([-135 35]);
axis('image');
axis('off');
colormap('hsv');
camlight('headlight');
material('shiny');
