How to import a local typescript module in bud.config.ts

I’d like to import in my bud.config.ts a module from a local typescript file named colors.ts. I tried the following, but none works:

import colors from './colors';
import colors from './colors.ts';
import colors from './colors.js';

I always get this error when running yarn build:

Cannot find module '/my_project/sage/colors.js'

What’s the correct way to do ?