To be able to paint you need to create a new image. While in Object mode
open the UV/Image Editor window. IN the Image menu, click on "New". Select the size of your image, usually 1024x1024 or 2048 x 2048.
When done, click on the name of the new image and give it a meaningful name, like "New Texture"
Click on the "pin" next to the image name to keep the image showing. Switch to edit mode, select the faces/maps that you want to paint, select the "textured" preview mode.
Go to you UV window, from the Image menu click on "Texture Painting". From the View menu, select Image Paint. IN the Image Paint panel click on "Draw", Pick a color paint, see it in the model, be happy :)

Paolo

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