The test of a true roan against a grey that looks to be a roan when young is said to be that a true roan remains the same in its summer coat all its life whereas a grey will gradually lighten each year as it ages.
Not sure what experience anyone has of this though certainly with the pony we had it was true - I first 'met' him when he was 4 and belonged to a friend, he eventually came to us and when he retired went to someone else I knew - he lived to be very old and his summer colour was pretty much constant
My roans have a different amount of roaning every couple of years...My QH has gotten redder with less silver/white mixed in then his previous pictures as a younger horse...the tone of his red has changed as well, he was a DEEP dark red when he was younger, now he's more of a orangey red chestnut :(
Roans often do one of two things: they either lighten or darken. When they lighten up they are called varnish roans, but your little guy seems to be doing the opposite. Roans change every year with the shedding of their winter coat, but they tend to keep the color of their legs, manes and tails the same while their bodies change. A little off topic, but your little shetland is very adorable :)