Calculating an index for color changing in a sequential "stream" of images.

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Hi all.

I'm building a simple device for my lab which actually captures images at specific intervals. Here is a sequence of images for example.

While scrolling down, you will notice that the color of the liquid inside those tubes is changing. So what I wanna achieve is to programmatically calculate an index that corresponds to the difference between the liquid colors in the two cases. I've never before dealt with image analysis and thus I would like to hear your opinions.

A first scan on the Google revealed the OpenCV library for python and seemed pretty famous. Also as a first step, I thought to extract the most representative (using coordinates) areas of each image and then do the comparison. Is that feasible?

Any ideas/tutorials/hints are welcome.

Thank you.