Dot spread rumours that Colin had Aids when she caught him and Barry in bed together but given time they became close friends.
Spoiler
From Wikipedia;
On 25 August 2016, it was announced that Cashman (Colin) had reprised the role for two episodes to air the following month. Colin will share scenes with Dot Cotton (Brown), but full details of the storyline were initially held back. Of his return to EastEnders, Cashman stated: "It was a real joy, indeed a privilege, to return to my old home of Albert Square. To be amongst so many friends again, and to be back in the place where 30 years ago I started an amazing journey. An amazing journey which incredibly helped to change the country, and certainly its attitude to lesbian gay and bisexual people."