Magnesium deficient plants have interveinal chlorosis with green mid-ribs. Leaf margins become yellow or reddish-purple. 

Magnesium deficiency in alfalfa 
(photo courtesy of PPI)
spotted alfalfa leaves from Mg deficiency
Magnesium deficiency in corn 
(photo courtesy of PPI)
brown and yellow margins and green veins on Mg deficient corn leaf
Magnesium deficiency in wheat 
(photo courtesy of PPI)

Magnesium deficiency in tomato with characteristic leaf curl 
(image by P. Gilabert, IPNI)

Mg deficient tomato leaf