"Yellow flesh" refers to the color of the inner part of certain fruits or vegetables. It means that when you cut or peel them, the part you see inside is yellow. Full definition
They have a dark purple skin and yellow flesh surrounding a large seed. (healthline.com)
I have scored these gathered nuts with a knife and roasted them — once in the oven, once on our outdoor grill — and eaten the pale yellow flesh with butter and a bit of cinnamon sugar. (firstthings.com)