Drag the under line upward.

Shader Editor

Add > Input > Color Attribute

Connect like this.

Or, like this.

Select Vertex Paint Mode.

You can draw,but it's only on the vertex.

Edit Mode > Edge > Subdivide

Increase the number of cuts.

Increase the number of cuts as many as your computer spec allows.

Now, you can draw on the surface.

Sculpt Mode is also nice to draw on the surface.