This is called the "Sonic Mania Plus Dye Sublimate Tee". "Sublimate" means turn something from a solid to a gas while skipping the liquid stage. And....somehow that makes the dye go onto the shirt better?

Whatever it does, it's capable of doing it in a really small way because those little colored dots you see there are actually repeating character faces for accusing Sonic, yelling Tails, looking Knuckles and...yelling Dr. Eggman? The tiny faces make the shirt look a little polka-dotted at a distance. The tee is about 25 dollars in the Sega Shop in 2018

Here's a new tee for fall season of 2018.
It's a dark navy blue, with a large classic forward-facing Sonic face on the front. The lines for the eyes, nose, mouth & ears are there, but instead of coloring him in normally, it uses a Green Hill Zone scene. There's water at the bottom (to keep the face mostly blue) the 'path' through the middle (where the eye-whites take up most of it) and blue sky for the top of the head. A Sonic sprite is seen jumping there kind of in the middle. The graphic is 16 bit style.
This combination of art & screen shot makes for an interesting theme for the shirt. Certainly, it's not been done before & is totally classic looking without being boring. This may be an Amazon item, at 19.00 in adults sizes.

This shirt is only available at Pacific Sun / Pac Sun!
It's a store generally found in shopping malls that has fun tees, shoes, school bags & other accessories. Sometimes they will obtain an exclusive item...and this one is quite nice! It uses the Sega CD era Japan art for Sonic in a sort of '3 point' stance looking concentrating. He's on a black/white checkered background, surrounded on 3 sides by Japanese words. The top word is his name. But what is the left edge word & the bottom word? (Write in if you can translate for credit!)
But look there in the bottom left corner, there's a little circle containing cute neo classic waving Tails. Everything here is in full bright color. The Tails detail is small, but the orange adds a fun splash of color to the design. This is an example of a nicely designed tee that doesn't resemble anything else while still being classically cool. This is an adult size item in 2018.

You've seen a dark grayish green tee shirt before...
So here's a close up of its small chest side 'patch' graphic / logo. The whole shirt's photo makes the logo too hard to catch the details on. You'll see this design scattered around on bunches of SegaShop Sonic Forces merchandise. It's an elongated oval with ring at the top, emerald at the bottom & Sonic face in the middle. The face is classic, and this time the eye is filled in (usually it is left background-color) The background is slightly 'v' shaped stripes, meant to evoke a feel of a military patch. It says "Special Operations Unit" in curved lettering around the oval.

Because a military or 'boot camp' theme was created to market Sonic Forces, something like this (and it's other military similar designs) are common in the SegaShop in 2018.