When two different colours overlap , fall on exactly the same spot, dsin(angle) is the same for the two wavelengths. n1/n2 = 2/3 or 3/4 or 4/5 etc since n is a whole number. Some of the visible wavelength that would be likely to satisfy this condition could be 400nm and 600nm , 450nm and 600nm , 400nm and 500 nm.
Take the first set : 3rd order 400nm and 2nd order 600nm fall at same angle.
second set: 3rd order 600nm and 4th order 450 nm ,
third set: 4th of 500nm and 5th of 400 nm .
