Applying RGB vegetation indices

Hello, I'm trying to applicate RGB vegetation indices into an drone image to extract vegetation cover, I am using an image and method from (https://www.sciencedirect.com/science/article/abs/pii/S1618866715000333) this paper to practice it. I tried to applicate the NrbVI equation mentioned in the paper to the image in this paper to acquire the same results they got. But was not sure whether this is the correct way to do it since I am new to matlab and wanted to get some advice.

If the result I got is correct, then I wanted to ask how I could do statistical analysis on the pixel values. Such as,

'RGB values are individually analyzed in terms of histogram, and their probabilistic distributions are fitted using the Gaussian functions. Their parameters of mean u and standard deviation can be calculated. The pixels lying outside of the range of u + 3 then will be excluded. In theory, this action can increase the ratio of color-channel consistency, and vegetation cover can be identified with higher accuracies.'

Origin = imread('image1.png')
Red=im2single(Origin(:,:,1));
Green=im2single(Origin(:,:,2));
Blue=im2single(Origin(:,:,3));
NrbVI1=(Red./(Red+Green+Blue)-Blue./(Red+Green+Blue))./((Red./(Red+Green+Blue)+Blue./(Red+Green+Blue)));
double(NrbVI1);
imshow(NrbVI1);
imhist(NrbVI1)

<- image1 from a paper https://www.sciencedirect.com/science/article/abs/pii/S1618866715000333