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)
Alfalfa leaves deficient in Mg with spotted leaves
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)
Wheat leaves with yellow interveinal chlorosis from Mg deficiency

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

Green tomato leaves with yellow interveinal chlorosis from Mg deficiency