Paint canvas background for tilted image?

[Joe Foe Toe]

Image here: TUAry0.jpg (1856×1080) (snipboard.io)

So shot was taken at 45-degrees,  leveling and cropping leaves four white triangle corners.  I suspect there's no way to color these corners in LR - it would make life easier - but I thought I'd ask here before I head it to Photoshop.

[Rob_Cullen · Accepted solution]

You suspect correct. Lightroom (being a parametric editor) does not recognize any pixels (transparency) in the blank areas, so it cannot change or color. Photoshop is the goto.