In a small saucepan, whisk flour into milk until smooth. Heat, stirring constantly, until very thick, thicker than pudding.
Remove and allow to cool fully. Placing the pan in an ice bath will speed up the process.
Beat butter and sugar together until fluffy in mixer. Add vanilla extract.
Once cool, add heated mixture to creamed butter mixture and beat until fluffy. When finished, it will look like thick whipped cream.
Divide filling between four bowls and tint one for each of the House colors. Fill one piping bag (filled with a round tip) with each color.
Stick piping tip into each cupcake and fill until cupcake puffs up a bit. Alternate colors as desired.