How do I make certain portions of an image white which I don't intend to use further in my calculations?

So the images I have attached are for reference (to make you understand what output I want). Theory: non-soil pixels were removed by selection of pixels with high ratios of green or blue to red (ratio greater than 1), or that had been identified as belonging to the colour correction card. This is what I have found in a published work. Can anyone please help me with what MATLAB code should I use to get similar output as in 'Capture2' given my input is 'Capture1'? Please and Thank you.