Hey, this is definitely a tough question.

I would say that a good ideal would be to narrow this down to A and B (green and yellow), and getting yellow is not an unreasonable thing (I would say most people, myself included, would answer this on reflex).

In terms of why green:

For this array, it is true that we would get a variety of dots on our array, some green, some yellow and some red as there are multiple types of cell samples.

The key here is that they ask for the colour of the region of the chip containing probes for the insulin gene, which is upregulated in the second sample.

This region of the dot has the highest affinity to bind to green labelled insulin RNA, so if there is a lot of green labelled insulin in the sample, it will out compete any red mRNA, which will wash off that part of the chip, leaving the dot completely green. That is, although you have a mix of red and green, the binding affinity and amount of green mRNA is what will affect the colour of the dot.

Therefore, we see a gene chip more like default red, and as there is more mRNA of your gene, the red will get outcompeted. For example if insulin was not upregulated by eating, there might only be enough insulin to kick off 50% of the red mRNA, and you would wind up with yellow.

Please let me know if this is unclear, it's a bit of a more subtle argument.