Commonly in the south of Vietnam "chanh" refers to a lime, and is green. You rarely find lemons in Sai Gon.
Yep. Chanh is lime. I have never seen a lemon in Vietnam I have lived there 2 năm rồi
Is có necessary here?
As adjective, could it be just quả chanh màu vàng?
Or colors do not work as adjective?
Or maybe this would be "the lemon has yellow color", but we could use "quả chanh vàng" == "the lemon is yellow"?
Có í necessary because vàng is an adjective. Together with màu it becomes a noun. So we need có á a verb. (The lemon has the color yellow)