Here are the 3 videos Charanya found on how to make very clean line color changes vertically, horizontally, and even diagonally.  I hope they help you!

horizantal - https://youtu.be/_2MQ4zMyNhI?si=yF-UA7JP-qXTLcAg

Vertical - https://youtu.be/O37u8Mrxq1Y?si=ZylC3VXVkikidF0G

Diagonal - https://youtu.be/WU6AcaFIMPc?si=ah41i770ANHf-B5O