A latex/rubber dummy is a natural material and will change over time. The material is affected by sunlight and temperature which can accelerate the aging process and cause the surface to feel sticky. If you experience changes in the surface you should discard the dummy immediately and replace it with a new one. 

Why is the nipple transparent on one latex dummy and more opaque on another one? 

There are a lot of benefits to latex. It is a soft material and it is therefore more comfortable for children to use. Latex is also quite resistant to bite marks and can tolerate being stretched. 

Latex is a natural material and will change over time. Some dummy nipples stay transparent while others will become more opaque, e.g. as a result of the oxygen in the air. The quality and safety of the product is not affected by these natural changes. 

Please note that you should start using a latex dummy immediately and not store it for a long period of time before use.