The Canadian Buick Parts Catalogue for 1958-64 full-size cars reveals several bits of information about the 900-series trim codes which appeared only on cars built in Canada. The attached catalogue page shows which body styles were built in Canada (certain higher-volume Special and Century models). The rest were imported from the US and employed US trim code nos.
The 4439 Special was available with 3-standard trim level codes using 400-series US nos. The 7-optional Custom trims were fancier, had carpet instead of rubber mats, and used bettter padding in the seat cushions. These Custom trim codes were in the 900-series and included 978.
On Special cars, the main color families that year were green, blue, and grey/black/white. There were a few choices of rust, beige, and turquoise. For color-keyed interior parts, code 978 used grey, black, or white items. The neutral colors would harmonize best with the rose exterior colors. The exact colors of the seat material could not be determined from the parts list. Most likely the seats were a grey or black fabric with white bolsters of Cordaveen (vinyl). The nearest US codes would be Custom trim 422 or standard trim 431.
The headliner, sunvisor, and some minor trim items were changed during the '58 model year from cloth to coated fabric. Early cars using code 978 had grey cloth in these areas. Late cars had ivory coated fabric.
In any case, the result is a beautiful car.