func (f *Field) flushingColor() ebiten.ColorM {
c := f.flushCount
if c < 0 {
c = 0
}
rate := float64(c) / maxFlushCount
clr := ebiten.ColorM{}
alpha := min(1, rate*2)
clr.Scale(1, 1, 1, alpha)
r := min(1, (1-rate)*2)
clr.Translate(r, 0, 0, 0)
return clr
}
