The code on the firewall is listed on the manufactures paint chip sheet along with the manufactures proprietary code and sometimes the mixing formula.  GM of Canada codes were common to all makes (Chevy, Pontiac, Olds, Buick).  For 1935-1955 GM of Canada used a paint code scheme where the first digit of the paint code was the last digit of the model year and the rest identified the specific paint color.  For years that ended in a 0, like 1940 or 1950, the leading zero was omitted from the paint code.

I've attached a chart for GM of Canada passenger car colours for 1955.  If your firewall tag says code 551 (Lancer Blue Metallic), that translates to C-I-L paint code Cilux 2352-1129 or Cilco 2221-1129.

You didn't specify the year of your car, so 640 could be from either 1936 or 1946 assuming your car was built in Canada.  I don't have the paint codes for those years, but if you contact Ken McGee and tell him you want GM of Canada passenger car paint chips he should be able to provide you something that will tell you what code 640 is and then you can share it with the rest of us.