What was the process of adding the dialog?
Initially Hergé's art was lettered directly onto the page of original art. It meant that if another language edition was to be made, that the content of balloons had to be covered over, and new dialogue added.
Over time, it was easier for Hergé to leave the balloons blank on the final art, and the dialogue to be inserted from a separate sheet of art, made into a printing plate; you can speed up production time in this way, by printing all the elements that are the same in one pass, then swapping text plates to over-print the artwork.
did he specify the colours, or did someone else decide them?
He was very particular about colour, and took great pains with it; although the initial colour albums used some painterly effects in their schemes, over the years he sought to get the colour as flat and uniform as possible.
As a note of particular importance for yourself, Hergé didn't colour the original art for publication (although he did colour it by hand from time-to-time, to give as gifts to friends and coleagues), but used copies, reduced to approximately publication size. This makes it far
easier to revise, rework and reproduce the art.
Initially he coloured art himself, assisted by his first wife Germaine; later he added specialist assistants, leading to a team at his studios, led by the head of department, Josette Baujot. Hergé's second wife, Fanny, was part of the team.