The comment for the "left bar" focus indicator recently merged to core in the style guide (http://tools.wmflabs.org/styleguide/desktop/section-1.html) is "When focused a progressive highlight appears to the left of the field."
But the hardcoded color for the left box-shadow in resources/src/mediawiki.ui/components/inputs.less is #5088f7, which isn't colorProgressive (#347bff). It's lighter and more washed-out.
Flow originally used #347bff for the focus indicator. Shahyar's prototype of new UI ideas used #5088f7, Flow switched to that value in April with template rendering, and that's been the color ever since. FWIW both the .png in the Trello card https://trello.com/c/8FOi1X1x/16-basic-input-field and the original Agora_specs.pdf use #4C85FB without actually specifying a color.
If we want a different color for the focus indicator we should derive it from colorProgressive using LESS color operations, and modify the comment in the CSS.