The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/63080ffcb680430e76247c3ec194f05f2092ff28

commit 63080ffcb680430e76247c3ec194f05f2092ff28
Author: Toni Barzic <tbarzic@google.com>
Date: Mon Dec 18 22:22:44 2017

Fix lock screen note launch button background color

Alpha is the first arg of SkColorSetARGB, not the last (as it was
used in this case). Also, given that the alpha is 0xFF, SkColorSetRGB
can be used instead.

BUG= 792126 

Change-Id: Ib93a9315dae807ec13db412b020f536e5a19d6fe
Reviewed-on: https://chromium-review.googlesource.com/832932
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524834}
[modify] https://crrev.com/63080ffcb680430e76247c3ec194f05f2092ff28/ash/login/ui/note_action_launch_button.cc

This bug requires manual review: M64 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Can I have some more context as to why this is a P1? Is it blocking functionality, or just cosmetic?

It's cosmetic