Why are the Euphorbia leaves turning yellow and wilting ?

California, United States

Updated Q with Photos :

Euphorbia leaves turned yellow.

Watered thoroughly thinking that it needed more water. Now after 2-3 days more leaves have turned yellow and are wilting.
Removed the aphids with spray, still some are on in another plant, Achillea.

What is causing the leaves to turn yellow.
How to reduce the salt buildup ???

Thanks.
Ph # 1 : taken on Sat 11th.
Ph # 2 : taken on Tues 14 th.

Old Q : Euphorbia leaves have turned yellow/dried around the lower part of the stems. Upper leaves are green. Aphid like bugs are on flowers.
Is the yellowing because of Aphids?

Thanks.