Make the lavender cold foam: Add the heavy cream, milk, lavender syrup, and food coloring (if using) to a small jar or container. Froth for 10-20 seconds with a milk frother, until the mixture is well combined. Set aside.
Add the water to a small microwave-safe jar or glass, and heat until boiling (30-45 seconds in the microwave). Sift the matcha into the hot water, removing any large clumps. Whisk the matcha and water together until smooth and slightly foamy. Set aside.
Add ice to a tall glass, then top with the oat milk and vanilla syrup. Stir to combine. Add the matcha and stir again.