Use the Color Thresholder app on the apps tab of the tool ribbon. I'd use hsv color space. Then adjust the thresholds and Export the code into a new function m-file.
rgbImage = imread('Captur11.jpg');
subplot(2, 1, 1);
imshow(rgbImage);
[BW,maskedRGBImage] = createMask(rgbImage);
% Fill holes
BW = imfill(BW, 'holes');
% Take largest blob.
BW = bwareafilt(BW, 1);
subplot(2, 1, 2);
imshow(BW)
function [BW,maskedRGBImage] = createMask(RGB)
%createMask Threshold RGB image using auto-generated code from colorThresholder app.
% [BW,MASKEDRGBIMAGE] = createMask(RGB) thresholds image RGB using
% auto-generated code from the colorThresholder app. The colorspace and
% range for each channel of the colorspace were set within the app. The
% segmentation mask is returned in BW, and a composite of the mask and
% original RGB images is returned in maskedRGBImage.
% Auto-generated by colorThresholder app on 11-May-2020
%------------------------------------------------------
% Convert RGB image to chosen color space
I = rgb2hsv(RGB);
% Define thresholds for channel 1 based on histogram settings
channel1Min = 0.584;
channel1Max = 0.619;
% Define thresholds for channel 2 based on histogram settings
channel2Min = 0.856;
channel2Max = 1.000;
% Define thresholds for channel 3 based on histogram settings
channel3Min = 0.000;
channel3Max = 1.000;
% Create mask based on chosen histogram thresholds
sliderBW = (I(:,:,1) >= channel1Min ) & (I(:,:,1) <= channel1Max) & ...
(I(:,:,2) >= channel2Min ) & (I(:,:,2) <= channel2Max) & ...
(I(:,:,3) >= channel3Min ) & (I(:,:,3) <= channel3Max);
BW = sliderBW;
% Initialize output masked image based on input image.
maskedRGBImage = RGB;
% Set background pixels where BW is false to zero.
maskedRGBImage(repmat(~BW,[1 1 3])) = 0;
end
