These 6 shirts from 2018 in the Sega Shop are quite the variety of designs & themes. These tees are each a part of a different theme the shop ended up going through later.
The first is a dark brown with the "Colorless Motion" theme that appeared on several other items. It's modern Sonic with a scattering of different font words to describe him as the background. The design is all white.
The 2nd & 5th tees are variants of each other with one gray & the other blue. They have Metal Sonic's head in a circle with his name & the slogan "Built For Speed" in it. (MS later went on to have no less than 6 solo shirts)
The 3rd tee is from Sonic Forces in the 'patchwork'

theme that it had. Although...it's just the shark/eggman symbol with "Eggman Empire" spelled out around it on an all black tee. The 4th is "Emerald Club", but it's labeled as 'emerald diamond' for some reason? It has Sonic & Knuckles heads & the Master Emerald in the middle, in a 'faceted' but simplified look. (Emerald Club was used on several other items, but the style here is reminiscent of the 'color shock' designs that would come later)
The final tee is an all black one with red checker background & SA1 'Tail holding Tails' as the design. It also has his name spelled out (both ways) & some little stars. They later went on to release many other solo-Tails tees, as the store seems to spotlight each character with a line as time goes on. Each tee here is about 25 dolalrs, in adults sizes.