The theory behind this is little bit more complex but try to get the idea of it by this following diagram by the Aresa.
Credit for this diagram should go to aresa
Usually tobacco plants only produce this pigment in their flowers changing their colour to pink but genetically modified plants will get stimulated by compounds in the soil and change their colour of leaves due to induction of red pigment production
The theory behind this :-
anthocyanin ( a red pigment ) in the presence of break down products of trinitrotoluene ( a bit harder to pronounce ) also known as TNT. TNT is an explosive often found in landmines. The involving product from this break down is nitrogen dioxide ( NO2 ) gas.
As you also know that to bury a landmine it just costs only 3 dollars but to de-mine them it costs more than 2000 dollars per mine.
In presence of landmines the colour of the leaves of the GM tobacco plants will be changed from green to orange after about 3-5 weeks after growth.
I hope you got the idea of this process and please be kind enough to make a comment about this post.