Polymer Baking Clays

First try in making polymer clay but I realized my white clay turns yellowish and there is a lot of air pockets :( Any way I can overcome this issue? I'm using

Polymer Clay Set 48 Colors Modeling Clay Sculpting and Oven Bake Kit Baking and Molding

12/42 Colors Polymer Clay Starter Kit DIY Soft Craft Oven Bake Clay Baking Modeling Clays Accessories And Storage Box Kid Gift - Realistic Reborn Dolls for Sale

FanFlex: Lightweight flexible crests and patches that fold easy. - FanTex: An all-new super comfortable soft fabric. - FanFit: The authentic on-ice

NARA Polymer Baking Clay, 1 Color, 1 Block, 55g (0.121 lb.) (Primary RED)

Sargent Art Polymer Beige Baking Clay, 1 pound block, Easy to Use

Sargent Art Polymer Gray Baking Clay, 1 pound block, Easy to Use

FIMO Effect oven-bake polymer clay, silver (metallic), Nr. 81, 57 gr

Sargent Art Polymer Gray Baking Clay 1 pound block Easy to Use and

Sargent Art Polymer Baking Clay, Black - Yahoo Shopping

Sargent Art Polymer Gray Baking Clay, 1 pound block

Tips For Baking Your Polymer Clay Projects – Sculpey