TM Nations (Forever) does not send out any
custom content. This can be a 3D vehicle change, a painted 2D skin, a funny horn, animated avatar, or presents for Santa...it just won't send them. However, it can receive
custom content from players using a registered TM United (Forever) account. As TMUF has no such limits it can send all custom content to everyone.
Car shows to other players as a Stadium (and that's the the only environment of the 7 you have), and it's got your own country design = you're likely using TMNF.
Cars shows as a default environment type, but appears as grey to other players = you're using a registered TM United account. Either your router ports and firewall aren't configured, or the custom content you are using hasn't arrived to be seen by other players (host the custom content remotely with a LOCator to cut this delay time down).
It's really been more FULLY detailed in thousands of other posts.
....use forum search if you need to know more.