In this article I want to give you some tips on why your Mari program refuses to let you paint your texture.
1. Click the view current paint target mode.
Sometimes you have selected the wrong layer to paint on, for example an procedural layer. Viewing your current paint target let s you be certain of the current layer at all time.
2. Check if your brush painting mode is set to "CLEAR"
Clea would act like an eraser to an already existing texture. Be sure to put it to the default setting if you want to lay down some paint.
In the projections tab :
3. Enable the mask Preview
If your whole object is one solid color your object or your channel are locked. Unlock them!
4. Projection on all
Check if you have set your projection to "selected only" and set it to "all".
Otherwise you are not able to paint as long as you haven't selected anything.
5. If all above doesn't help
Re-select your brush blending mode.
If you have some more tips, let me know!