My employer provides a Mac laptop with the Office suite. Red flag, green flag, or yellow?

Word, Excel, and arguably PowerPoint are still the best tools im their respective classes, so if you mean those then very much a green flag.

If they're also making you use Outlook or especially Teams then they're going to start losing "points".

My workplace let's me choose Mac or Dell laptops.

What if they provide both?

My calculations tell me that would be a yellow flag.

My knowledge of colors tells me red and green make brown.

#ffff00 is a pretty bright yellow color.

What does a brown flag tell us?

proceed with caution

being provided a laptop is a red flag...? unless you get hp or cheap dell, then yeah red flag

No for me both Microsoft and Mac devices are a red flag.