Write some custom code - php - wordpress

I have an wordpress+woocommerce eshop about clothing, and I would like to (using wp all export) to export on xml all products with product variations but only different variations by color and not by size. What I have done so far is to install plugin named: "show single variations" and then choose which product variations are visible in catalog. Then using wp all export plugin and "_visibility" tag I have managed to export all products that are visible on catalog to xml. The problem is that some of my size and color tags on product variations are empty. I have replicated my site to help you better understand here : [login to view URL] You can check export 3 So I would need some custom code to do these things: 1) IF Product Size is empty then take value from parent's id product size 2)In product color: export only the first color. For example if it has 3 variations Blue, red and white only export the first one (white) 3) There is no custom field that includes the discount, The formula is this one: ( {Regular Price} - {Sale Price} ) / {Regular Price} ) *100 Could you help me write the function? 4) If a variable product has attributes, e.g. color and size, but only size is used for variations then color tag in variations is empty. If empty, can we export parent product's color tag to variations? For example a dress that has attributes: Color:Blue and size: S,M,L, only size is used for variations so there are 3 variations, which have color tag empty, I would like to use parents color in the variations, is it possible? Thanks!