Assigning material and colouring

First set the mesh to object mode.

Then, right click and go to assign new material.

After that, click phong, and select the colour wanted.

Then switch to face mode.

Choose the surfaces that designer wanted to colour.

Right click again and select paint, 3d paint.

Assign texture for the surface.

Choose the colour that wanted and paint on surface.

For the project, 3d paint is skipped due to hanging and errors. The designer just used 'assign new material' by selecting faces for colouring the mesh.