What is a destructive interference?
This happens when two or more waves that are out of phase with a path difference of (n+ 0.5)λ, interfere.
Here two coherent waves (in indigo and pink), are in antiphase (phase difference of 180 degrees), interfere destructively forming a resultant wave with with no displacement (application of principle of superposition). This will only occur if the interfering waves are of equal amplitudes and are out of phase.
Here however, the waves are of different amplitudes and out of phase. By applying principle of superposition (1- 0.7 = 0.3 displacement units), the resultant wave has an amplitude less than that of any individual wave, but not zero as in the previous case.
Destructive interference can be observed as dark fringes (minimas) in case of light interference or flattened waves in case of water.