The typical signs of magnesium deficiency are interveinal chlorosis. In mild cases the lower leaves may become pale in colour. Where magnesium is present to excess then lower and middle leaves become yellow with veins and margins remaining green at first, reddish/purple areas may appear on the older leaves and some small brown areas may appear between leaf veins. Flower size is reduced and colour may be paler than normal. 

Remedial Actions
For deficiencies dissolve epsom salt in water and apply as a foliar spray.

For excess water heavily in an attempt to leach away excess magnesium.