Plus there's zero direct evidence for their colours so there's no option but to use guesswork in these cases.
And a lot of dinosaur reconstructions may be more for edutainment value rather than reflecting a scholarly best-guess. There's no uniform methodology across all these disciplines.
This is no longer true! Starting with Sinosauropteryx in 2010, paleontologists have identified what they believe to be fossilized melanin-containing organelles. These organelles, called melanosomes, have different shapes depending on which color they produce, and those shapes are preserved well enough to be visible under an electron microscope.