you will need blender so make sure you have it installed or are installing it

Once you have blender upload the model you want to use or create one then

now change object mode to vertex paint and you can change the 2 colors at the top to your likings and paint your model


After you paint your model go back to object mode, select the model, go to file then export and make sure you check off color on the right side of the export menu

Make sure to create a models folder just like this, the name should be "Model" Image( https://ibb.co/h2YcgbJ) then you can drag your obj file into the models folder. Please make sure the model you upload is a obj model.