It would actually be a fourth color, as there's already three (or four if you count white): yellow for partial sets, blue for a full set (1x of each card), and green for a full playset (4x of each card).
You know what would be even better? Being able to set your own custom collection goals per edition. I could imagine this being a fourth column next to wishlist. You would simply add however many copies you want of each card in a given edition, and then the color coding on the Editions page would reflect whether or not you've completed that collection goal or not. I recommend a 4-color system -- white / no color for N/A (no collection goal set), light red for 0% (collection goal set but no cards collected yet), yellow for 1%-99% (partial completion), and green for 100% (full completion). Or just combine 0% with yellow for a simple 3-color system.
This could tie into the existing Wishlist as well. Users could optionally enable an auto-wishlist setting, in which all collection goals that don't have adequate inventory counts to cover them get auto-added to your wishlist. And then when you add one or more copies of that card to your inventory, it gets auto-decremented in your wishlist.
And the Mass Input convenience methods could be extended, not only to include the new Collection Goal column (and probably a Wishlist + Collection Goal option), but also to allow for partial collector number ranges for adding the cards. Then you as a user can easily dictate, "I want 4x copies of the main Eldraine set not including basic lands (#001 - #249), and 1x copy of the showcase cards (#273 - #302). But none of those stupid extended art variants!"
You've got my vote! 
Excellent ideas.