The specific shade of blue varies from mill to mill, from weight to weight, and from colour scheme to colour scheme.

There are a number of available colour schemes such as modern, ancient, weathered, muted, etc.

Each mill's "modern" will have slightly different shades.

And the same mill's colours/shades might vary a bit between their 13oz and their 17oz fabric.

So yes you will need swatches.

People often don't notice, by the way, how much purple is present in the colour we call "navy blue".