For flat colors I tend to get that part of the job done pretty quickly.
Around November(?) I helped with some of the character coloring for an animation called Chronexia and the Eight Seals on Youtube (Sadly the entire episode didn't do so well) Anyway, the director taught me and the other guest who were helping out a neat trick to help us quickly get the base colors in.
I will note that I use Clip Studio Paint Pro, so I'm not sure how this is done in other programs.
Having The Area Scaling for the bucket tool set to 14 helps so that I don't need to worry about a white/uncolored gap between the line art and the colored area. Multiple Referring is set to all layers helps so that you can still select that area when using a different layer. You can even do this with the magic wand when importing images like textures but I'm personally not a fan of the wand when doing that, still misses some spots.
I just use the Marquee/lasso tool to select a small area, import a texture/tone and use the bucket to fill the rest.