Matching colours can be very difficult even with a colour code chart - but a basic tip is that you will need at least 2 of each colour - say a dark blue and a light blue ( dark blue is not blue with black added). a good example of what im on about is that people will refer to a deep red as opposed to just red - these are 2 different pigments, so you need deep red and standard or almost fire engine red, also canary yellow and a yellow ochre - a standard green and a purple, black and then white - from this you can mix almost any colour.
If the BG isnt going to be illuminated then you really only have to do half the job. BG inks are transparent and so not only do you have to match the colour but also the level of transparency.
Another tip is not to use standard acrylics as they dry darker - use enamel or acrylic lacquer.
If you cant match the colours well then the scrape away and replace method is best... at the same time though, with just cracking like you have, if you can match the colour then you can just run a sable haired brush through the crack..
oh yeh... black lines first on a BG.... and GOOD LUCK!!!
