There is another established (not proposed) system of signs for Solresol: communication with the help of (maritime?) flags. I’ve read about this type of signalization here.
So, if I understood right, there was such system of correlation:
do — one-colour white flag;
re — one-colour blue flag;
mi — one-colour red flag;
fa — two-colored flag: on top — white, below — blue;
sol — two-colored flag: on top — white, below — red;
la — two-colored flag: on top — blue, below — red;
si — two-colored flag: on top — red, below — blue.
Also you can see this picture from the above-mentioned source:
So, if I understood right, there was such system of correlation:
do — one-colour white flag;
re — one-colour blue flag;
mi — one-colour red flag;
fa — two-colored flag: on top — white, below — blue;
sol — two-colored flag: on top — white, below — red;
la — two-colored flag: on top — blue, below — red;
si — two-colored flag: on top — red, below — blue.
Also you can see this picture from the above-mentioned source: