Had this idea last week when the latebloomer flags were going around. Not a ton of thought into the choices of colors, tbh, I just thought it would look good and kind of follow the looking back/looking forward motif of /u/Kendota_Tanassian's versions. But with a little Seychelles vibe.
Side note, if I can upload my Illustrator file, can someone explain why I'm getting the half-pixel white line along the purple-grey seam? Stroke is set to 0 and the grey is a solid rectangle arranged all the way back, I can't figure out why the white is showing up but it shows in Ai as well as png and svg renders.
My Ai file is here: https://gofile.io/?c=AJakY5
You can use any art or image editing program you want that let's you make custom shapes with custom color pickers. But these are the ones I reccomend or use(d).
Adobe illustrator or photoshop. But illustrator is better for design since it's based on vector graphics (raster graphics is better for drawings. Vector is better when you're making stuff like logos and etc since you're going to be sizing everything differently and it needs to be sleek and easily editable).
Photopea is a browser based and open source photoshop alternative that's free. But if you have adblock on it will stop a lot with "wE sEe ThAt yOu HaVe AdBlOcK oN..." pop ups.
Gravity designer is the Illustrator alternative I have used in the past. It's very annoying and the web version (iirc I think there's a version you can download to your PC?) is buggy but it's free (though it has a premium version with stuff locked behind paywalls) and gets the job done with simple shapes. Which is all you need for a flag. I used it to make this flag for one of my nations in nationstates.
I hate gimp and I don't recommend it.