Thank you in advance for your help - I have spent hours trying to resolve the following: I opened a JPEG in Krita and applied a Color to Alpha filter to isolate the drawing of the JPEG (the drawing/lineart is now isolated, with the background being checkerboarded). How do I fill the checkerboarded with color? or draw? I can’t do anything (the brush does not respond or any other functions):.if I create a new layer, the drawing is also on that layer, which also does not respond to any modifications… or is there another way to extract the linear of a JPEG? This is quite frustrating…Thank you…!
Hello and welcome to the forum 
What you seem to be trying to do should be straightforward but there are various different ways of doing anything.
It would be very useful to see a screenshot of your entire krita window, with the Layers docker visible.
Now that you’ve used Colour to Alpha on the original .jpg image, you’ll have the lineart isolated on a transparent layer, through which you can see the checkerboard ‘ultimate background’ (which doesn’t really exist but is put there to let you know that you’re looking through something that is transparent).
Now that you have the lineart on its own transparent layer, the simplest way to give it a background colour is to put a new layer under the lineart layer and fill that ‘background layer’ with the colour of your choice, using the Fill tool.
Which layer have you put an alpha lock on and why did you do that?
If you leave the lineart layer alone, you don’t need to put an alpha lock on it. If you want to ‘protect’ it, you can lock the layer itself to prevent any changes to it.
If you want to overpaint the lineart itself then yes, putting an alpha lock on the lineart layer is a way of doing that.
I suspect that when you create a new layer, which will be transparent when first created, you are seeing the lineart layer through the new transparent layer.
If the brush does not respond, is that because you’ve put alpha lock on the layer you’re trying to paint on?
If you create a new document, can you paint on it?
Dear Ahab,
Thank you so much for your detailed answer - I can’t tell you how much I appreciate it 
As you have correctly stated, I have used color to alpha to isolate the lineart on a transparent layer (LAYER 1 on the picture, which is where I put the color to alpha filter). Is this what you would have done to isolate the lineart?
Next is my issue: when I create a new layer, it looks exactly as the first layer and I cannot paint nor do anything on this layer.
When you say “If you leave the lineart layer alone, you don’t need to put an alpha lock on it. If you want to ‘protect’ it, you can lock the layer itself to prevent any changes to it.”: I just want to clarify that an alpha lock is the same as the color to alpha filter? If you’re talking about the small lock on the side, I clicked on it to protect it before creating the second layer or any subsequent layers.
Then, when you say: “I suspect that when you create a new layer, which will be transparent when first created, you are seeing the lineart layer through the new transparent layer.” Absolutely!
“If the brush does not respond, is that because you’ve put alpha lock on the layer you’re trying to paint on?” I don’t think so as I just create this new layer and I can’t do anything on it. I just would to clarify the definition of "alpha lock " versus “color to alpha filter”, versus “the small lock”. In any event, I just create the new layer and don’t do anything else but it’s completely locked anyway.
If you could just tell me in a few steps what you would do from scratch with the Jpeg image in order to paint it in krita… Thank you so, so much in advance!
Warmly,
Waterwoman
Hi @Waterwoman,
The first thing I can see is: save your work as krita file format
Menu File > Save as… and then choose in Files of type > Krita document (*.kra)
Otherwise, saving it as jpeg, everything will be merged in a single image without any transparency and you’ll have to restart you work if you close krita…
You might have duplicated the layer (because of name ‘copy of layer 1’) using the duplicate icon
To create a new empty layer, in “+” menu, choose “add a new paint layer”
The fact you can’t draw on it sounds like you have a selection (can’t see it on your screenshot but may be outside of visible canvas area).
Ensure you don’t have any selection before drawing (menu select > deselect or shift + ctrl + A)
Grum999
You have quite a small screen. What type of computer are you using?
Are you painting with a mouse, a touchpad or a drawing tablet or some kind of touchscreen stylus?
You have the Freehand Brush Tool selected but no brush preset name on the status bar at the bottom left. I don’t know if it’s not shown for small screens. Maybe it isn’t.
Where is your Brush Presets docker?
Have you used krita before on this computer or any computer?
Have you used any painting application on this computer?
On the top, you’ll notice an eraser icon and the word ‘Erase’. This means that you’ve selected an eraser ‘brush’ to use and these don’t paint, they erase. Hence no painting action at the moment.
The question is, how did you select an eraser for use and how are you going to select a painting brush to use??
The advice from @Grum999 is very good, especially about Saving as a .kra file so you can continue work at a later time.
It does look like you duplicated Layer 1 somehow and you don’t need that duplicate so I suggest that you delete it by clicking on it to select it and then press the ‘trashcan’ icon at the bottom of the docker to delete it. (You can always, make another copy later if you really want one.)
An ‘alpha lock’ is something totally different that restricts the application of paint to areas that already have paint and it’s not something you’ve done or really need to do. (It’s the little cross/checker pattern icon by the side of a layer. Please don’t try it yet.)
The lock at the side is the layer lock and prevents any change to a layer. You’ve put that on your lineart layer, which is good, so you can’t accidentally paint/erase on it and spoil it.
All you need to do is click on Layer 2 and click-drag it under Layer 1 where it can act as a background layer that you can paint on in any way you like. You can put multiple new layers under Layer 1 to have quite a complicated background image that you make. Sky, grass, clouds, moon, logo, etc.
Hi Grum999,
It works, it works, it works! THANK YOU!!!
Have a rainbowy day,
Waterwoman
Good Morning Ahab,
I saved it as a Krita document, inserted a layer under Layer 1 and managed to deselect the eraser (it was the main issue) and…It WORKS!!! THANK YOU FOR YOUR KINDNESS, MERCI!!!
Looking forward to seeing your artworks!
Waterwoman
I’m glad you’ve got it all working but you may need some advice about tools and dockers and their layout in the future.
I’d like to recommend the manual which is very well presented and written:
For now, enjoy your new found abilities and do come back if you need any more help or advice