Traditionally Hollywood has showed us red shields for the Legions, but of course Hollywood only pays historical fact-checkers to say they have one, not to do what the fact-checkers say! The actual historical record is scanty at best until the Notitia Dignitatum, which shows hundreds of shield colors and designs for Roman military units in the 5th century. Prior to that we have a handful of shields and depictions of shields, mostly red with some hints at white, brown, pink, green, and yellow. We also know through reputable sources that Roman Legions at the time of Caesar and Pompey could be differentiated based on shields, though if this is because they were different colors, different designs, or both isn't clear. It could also be that, since it was a Civil War, one side or the other specifically changed their shields to avoid confusion in battle, like the USA and CSA armies early in the US Civil War. Lastly we have one source indicating that different cohorts had different shield designs within a legion to make command and control easier (I can tell it's VI Cohort because of the blue shields or cross design for example) but it's not clear if the color was different, the symbol was different, both were different, or even if the individual parts of the Legion retained some overarching Legion identifier. (So all red shields with different symbols, or different colors but same symbol, or same color and symbol with a secondary cohort symbol etc etc).
All that being said, the Legions themselves would exist for centuries. Tradition is important to soldiers and to Rome as a culture. Therefor it seems to me that, say, Legions in the 5th century that could trace their history back to Late Republican/Early Imperial Legions might still use the colors and symbols in existence then, passed down through the centuries. For example the regimental colors of the British army have been roughly the same for each regiment for 300 years. Has anyone seen or done any research on this? Is it even possible to associate Notitia Dignitatum shields to their proper legion and trace them back? Were there any Legions in the 5th century that we know COULD trace their lineage back so far?