Example build https://luci-logdog.appspot.com/v/?s=chromeos%2Fbb%2Fchromeos%2Fkefka-release%2F799%2F%2B%2Frecipes%2Fsteps%2FUprev%2F0%2Fstdout

Has lines that render like:
10:16:37: ERROR: Missing 9999 ebuild in /b/cbuild/internal_master/src/third_party/chromiumos-overlay/chromeos-base/autotest-deps-camera-hal3

The [1;31m and [0m look like escape sequences to colour the lines red/bold and then to reset it to normal.

Buildbot (https://uberchromegw.corp.google.com/i/chromeos/builders/kefka-release/builds/799/steps/Uprev/logs/stdio) appears to be rendering those lines like:
<span class="stdout ansi1 ansi31">