The below applications doesn't have an icon designed for its tray(panel). Please consider this.
Applications:

Why you again request about FlareGet? Icon for tray available and worked with hardcode-tray script. You install hardcode-tray and patched sni-qt?
Oh, sorry I didn't get your response. I'll edit it out.
But I applied using hardcode-tray and i have sni-qt latest version installed. But still the problem persists.
What distro you use?

I use Ubuntu 16.04 LTS with XFCE
See my screen, ssr and flareget use papirus icons on tray.
See you screen - icons not use icon theme. Because you NOT install PATCHED sni-qt.
sudo add-apt-repository ppa:andreas-angerer89/sni-qt-patched
sudo apt update
sudo apt install --reinstall sni-qt libqt4-svg
I installed the patched sni-qt and applied hardcode-tray again. But still it won't work.
I installed the patched sni-qt and applied hardcode-tray again. But still it won't work.
Sorry i'm not use XFCE, try open issue on hardcode-tray.
Gnome Pie now worked.

Easystroke not possible change icon, because icon used from binary file. But you can didsable tray icon on preferencies.
With hardcode-tray changed only flareget icon. ssr and gnome-pie worked in box with Papirus.
Also try qtconfig-qt4, maybe on your system not used GTK theme and icon theme for Qt4-apps.
sudo apt install qt4-qtconfig
Even qtconfig-qt4 latest version installed.
run qtconfig-qt4 Set GUI Style - GTK and save options on File-Save. Restart Qt-app and check
Still doesn't work (It is already set to GTK+ anyways) :(
Oh i'm remember this bug.
It's upstream bug xfce4-session:
https://bugzilla.xfce.org/show_bug.cgi?id=12012
It's happened because xfce4-session compiled with flag --disable-legacy-sm - that breaks qt4 integration on xfce.
I try compile without flag --disable-legacy-sm and Qt-4 apps looking fine and use icon theme. You can install this package:
64bit:
https://launchpad.net/~varlesh-l/+archive/ubuntu/test/+files/xfce4-session_4.12.1-3ubuntu2_amd64.deb
32bit:
https://launchpad.net/~varlesh-l/+archive/ubuntu/test/+files/xfce4-session_4.12.1-3ubuntu2_i386.deb
Install package and restart system.
Also if you use non-English language on OS maybe you have trouble with this package, because without --disable-legacy-sm not applied another languages - used only English. See this:
https://bugzilla.xfce.org/show_bug.cgi?id=13685
It says failed to install package. I mean the .deb file you gave.
sudo dpkg -i PACKAGE
the package installed, but still it won't work.
reboot OS
Yeah, I rebooted.
hm it's strange, for me this worked and qt4-apps use gtk-icon theme.
I don't know anymore how help... maybe try another distro, for example manjaro or another DE
Thanks for your help though. Much appreciated. :+1:
Now it works, But only SimpleScreenRecorder. Others don't work.
But simplescreenrecorder icon is colored at startup, and becomes white in next screens, and when i click back, it becomes colored. (Screenshot below)


sorry, but i don't know why... on Gnome all fine:

If you want good and not bugged DE for tray i recommend Unity 7, because mostly apps use appindicator and write for this desktop