Looking at the code you provided, it is not clear what this line is trying to achieve:
slice(:,:,i) = (temp(:,:,1)==255) + 2*(temp(:,:,2)==255) + 4*(temp(:,:,3)==255);
However, if your goal is to convert a color image to grayscale (thus eliminating the third dimension of your image data), you can achieve this using the rbg2gray function as follows:
slice(:,:,i) = rgb2gray(temp);
You can then view your image using the imagesc function.
In order to view a 3D representation of your data (as a volume), you can use the isosurface function and try different values for the isovalue input.
There is also a file exchange submission called sliceomatic that can allow you to visualize your 3D data and help you select the isosurface and slices in different dimensions.
You can try running the example below and see if this works for you (note that I renamed the variable slice to myslice because it shadows a native MATLAB function. Also, I added a few lines to initialize the variable myslice with the appropriate dimensions):
filebase = 'C:\Users\user\Desktop\matlab';
startFrame = 1;
endFrame = 26;
% Initialize slice below. If you know the sizes M and N in advance, you can
% skip the following two lines
filename=[filebase, num2str(1,'%2d'),'.tif'] ;
temp=double(imresize(imread(filename), 0.5));
[M,N] = size(temp(:,:,1));
myslice = zeros(M,N,endFrame-startFrame+1);
for i=startFrame:endFrame
filename=[filebase, num2str(i,'%2d'),'.tif'] ;
temp=double(imresize(imread(filename), 0.5));
% convert to gray scale
myslice(:,:,i) = rgb2gray(temp);
imagesc(myslice(:,:,i));
colormap('gray')
drawnow
end
% try isosurface with isovalue = 0.5
figure
isosurface(myslice,0.5)
% % Uncomment to try the sliceomatic function
% figure;
% sliceomatic(myslice)
