When there are only optional, non-security updates, the tray icon is a shield with green check. http://i.imgur.com/QGec72Z.png When there is no updates, it also shows this icon. (I keep it always visable in tray) So I do not know if there is updates available. I think "green check" tray icon gives a feeling of everything is well, nothing to do, or update has finnished. My suggestion is : No updates -> green check Optional updates, New feature updates -> blue plus (+) Security updates -> orange exclamation mark (!) Serious security updates -> red cross (×) Reproducible: Always
Problem is there's only 3 states: high, medium and low. Now, I don't know what software is using the icons as various package managers use them. I know Muon updater uses them so perhaps this is the case. Also, in the Plasma theme there's an element which does provide icons for these kind of notifications (software updates). Some of icons included in this item are close to what you're suggesting but as far as I know the software has to have some sort of integration with the system tray.
plasma5-pk-updates is the package name of software updater I am using. I think its maintainer might know something.
plasma-pk-updates is not currently a kde.org project (as far as I know), best to request of them at https://github.com/caybro/plasma-pk-updates