where fast/flash makes more sense and is more interesting for red than green. moreover, fast is better for defensive purposes than offensive. for offense, use haste for surprise attack.

most of green flash creatures overlap with blue effects, so it's mostly redundant to put flash as blue/green primary. instead red's fast/flash supports more tactical and defensive options than green. especially paired with the Guardian keyword, this is a fast blocker that surprises the attacking player.

I didn't get the comment above at all for far too long until remembering that fast is a typeline thing here. That's an issue. Without reminder text fast creatures might very well be in a place where their fast is too hard to notice.

In the nonrefreshed color pie I've always kinda assumed white to get a secondary nod of flash. And it would fit with the other two abilities on this card. I'd right now allow a monowhite card to have flash, double strike and the ability to block an additional creature.

Your monowhite would be great in nonrefreshed set. Since WotC's white color pie is quite dull and restrictive compared to other colors. But it doesn't fit in my refreshed color pie.

Fast is primary blue-red. So those are the colors if you want instant blockers.

Also double strike is black-red here.