Polyacrylate seals look the same as nitrile seals. They are elastomers designed for higher temperatures (to 3000F-3500F depending on the lubricant). They are also resistant to extreme pressure lubricants. Polyacrylate is often used for differential pinion bearing seals.
Silicone seals are often used in engines and transmissions. They are usually colored gray, red, orange, or blue. Silicone is a polymer that is very flexible and absorbs lubricant. Silicone seals are resistant to friction and wear. They can operate at temperatures from 3250F to 3500F depending on the lubricant used.