A lot of kits only send one color of rigging line. Standing rigging is always tarred and thus dark. What I do is use a black shoe polish. It also has the advantage of waxing the line so it does not fray.
However, the ratlines were not tarred and were made of hemp and 1 1/2" in circumference. To calculate the size of rigging line for your particular ship, use the following formula:
1.5 inches times 25.4 divided by 3.1415926 divided by the scale of your ship.
The Soleil Rayoal is 1/77 so the calculation is 1.5 x 25.4 / 3.1415926 / 77 giving mine size as 0.1575 mm.
If the size of rope is given as a diameter, you do not divide by pi.