I think the grey tint had been removed a long time ago. You shouldn't see it in any recent canary. If you have repro steps for how to make it grey, please reopen. 

Tapping on the search control in the bottom toolbar selects the current omnibox text with grey highlighting. 

Are there any global settings that may affect the color of text highlights?
My text selection is blue.
See video: https://drive.google.com/open?id=1-KGGm5c-kQGB4IZmiyLG55rCeKcKWJy2

Lindsay, if the test team encounters this, I'd greatly appreciate reproduction steps. We've seen this in the wild a few times, but we don't know how to repro. 

if the first time I focus the omnibox is on the NTP
then it will be grey forever
if the first time i focus is on a webpage, it'll be blue forever

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b3d2237468530603ebf702cdc993359e17ffe95

commit 2b3d2237468530603ebf702cdc993359e17ffe95
Author: stkhapugin@chromium.org <stkhapugin@chromium.org>
Date: Wed Aug 01 09:29:43 2018

Fix inline autocomplete highlight color.

Avoids caching the inline autocomplete color in UI Refresh, and uses
_displayedTintColor instead of self.tintColor (which can be transparent
when the textfield is hidden behind inline autocomplete or preedit).

Bug:  865497 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I358cd0b10694271d4d0514b3dfe9b7415c1c9106
Reviewed-on: https://chromium-review.googlesource.com/1152985
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579736}
[modify] https://crrev.com/2b3d2237468530603ebf702cdc993359e17ffe95/ios/chrome/browser/ui/omnibox/omnibox_text_field_ios.mm

This bug requires manual review: Less than 25 days to go before AppStore submit on M69
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), kariahda@(iOS), cindyb@(ChromeOS), govind@(Desktop)

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

Approved. Please merge asap.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3563687c183eaa06402e02efd3c459c366bc7445

commit 3563687c183eaa06402e02efd3c459c366bc7445
Author: stkhapugin@chromium.org <stkhapugin@chromium.org>
Date: Wed Aug 08 09:23:07 2018

Fix inline autocomplete highlight color.

Avoids caching the inline autocomplete color in UI Refresh, and uses
_displayedTintColor instead of self.tintColor (which can be transparent
when the textfield is hidden behind inline autocomplete or preedit).

Bug:  865497 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I358cd0b10694271d4d0514b3dfe9b7415c1c9106
Reviewed-on: https://chromium-review.googlesource.com/1152985
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#579736}(cherry picked from commit 2b3d2237468530603ebf702cdc993359e17ffe95)
Reviewed-on: https://chromium-review.googlesource.com/1163682
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#489}
Cr-Branched-From: 271eaf50594eb818c9295dc78d364aea18c82ea8-refs/heads/master@{#576753}
[modify] https://crrev.com/3563687c183eaa06402e02efd3c459c366bc7445/ios/chrome/browser/ui/omnibox/omnibox_text_field_ios.mm

Verified on M69.0.3497.41 beta
iPhone6s, iPad Pro
iOS 11.4.1

Autocomplete text is always displayed in blue color.