Any time you receive a voucher, the color is always chosen randomly. Since the vouchers are of increasing value (green > blue > purple > gold), some colors are rarer than others. The golden voucher, in particular, is a highly valuable collectable and is not easy to find!
Don’t give up though! If you keep finding and opening mystery boxes (both in your and other players’ farms) and completing boat orders and special truck orders, you will eventually find the vouchers you need!