Colorful leaves?

 My spinaches have grown well enough to be harvested. Most of them look healthy but some leaves are turning to yellow from the edge of the leaves as shown in the second photo.
 I think the yellowish leaves might be caused by the shortage of minerals like calcium and magnesium, which can create chlorophyl.  If you have any idea on that, I would like you to advice me.
I don't like "colorful spinaches". They should be dark green.



-- from iPad