I have one goal with colorizing. I try and make it so realistic that the final image becomes unremarkable--until you become aware the original was in black and white.
The details really count towards giving the impression of realism. For instance, something like the sclera [the white part of the eyes] is a giveaway. They are not brilliant white unless they've been retouched in Photoshop. When colors are too perfect, it's a sign of a colorization no matter how flawless the execution.
Lloyd advises amateurs trying their hand at colorization to pay close attention to how light affects the colors of certain materials, especially machine parts. He also suggests developing a thick skin: "The irony is that colorizers are accused of destroying history,” he says, “when in fact we have the utmost reverence for the source material."