Johan Kees
A Fixture
Painting a shield is not the same as painting a plaster wall though. So it's a chemistry question. Based on what I have seen so far, I have to assume Johan has it right.
As I wrote earlier about the intensity of the colour, all depended on the wealth of the person who bought a coloured item. The wealthier, the more intense dyed items could be purchased. Legionaries were not that well paid at all, so one can reasonably state that the colours used to paint their shields were of a lower quality than let us say, the dyes or paints used for amongst others the frescos in the homes of Pompeii's upperclass. All in all, would it be too far fetched to assume that legionaries used plant or other natural extracts dissolved in water, oils, etc., with as a result low chroma colours and rapid fading? I remember having read that, while on campaign, the English Victorian soldiers stained their white helmets with tea to make them stand out less. I can imagine that the Roman poor and soldier's class did something similar, meaning to use readily available, or makeshift coloured solutions.
Then about the shield designs itself. One must not forget that depictions of legionaries on triumphal arches and columns are always idealized images and also very likely interpretations of the craftsmen involved. It is not because there are shield designs visible on Trajan's column (completed in the early second century AD) that these were standard practice for the whole of republican and imperial Roman rule, spanning several centuries. Of course, absence of proof is no proof of absence, of that I am aware.
Nevertheless, IMHO it would not be unlogical to suggest that while on campaign, shield designs were the least of the legionary's worries as there were many, heavy camp duties to perform. Moreover, I cannot imagine that some legion's housepainter took care to restore damaged or faded designs of the many hundreds of shields in use. There simply would not have been enough time for it. As such, I stick to the idea that applying shield designs was not a common practice at all while at war, but that these might have been applied solely for the thriumphal marches in Rome once the engaged enemy was defeated. Hence the depiction of shield designs on the thriumphal arches and columns.
However, and with this I will rest my case, a miniature of a Roman legionary, centurion, etc. looks not complete without a nicely painted shield design. But then we are talking about artistry and idealistic views of the subject of our interest, not about historical correctness.
Jhan