Are you masking a footage layer or a solid? If it's a solid, change its color to white. If it's footage, you can put a white solid below it to fill in the hole.
If you're doing something else, you need to elaborate on what you're doing.
im trying to do this
i have a video
i want to hide a aprt of the video with a mask
so i select the whole video with the first mask
then i create a mask inside and i press instead of add i set less (i suppose in english is less)
so that part become black, but how can i change the colour?
First of all you need to understand what is going on when you apply a mask.
Mask cuts hole in a layer (guess, proper word to your current mask settings is 'Subtract').
If the footage on which you apply the mask is a single layer in your composition, you see the composition background in the 'hole'.
Therefore, you can either change your composition background colour or place a solid with desired colour below your footage.
If the footage is not the only layer in a composition, but you still want to see some solid colour inside a hole, whereas other things on a background remain preserved, you can e.g. duplicate your footage with mask, replace lower instanse with a solid (painted to your favourite colour) and invert 'Subtract' to 'Add'.
hole, the word that makes me understand XD i got now why i can't color inside the hole but i have to color what i see in the hole (so the background)
can you explain me the last part? if the video is not the only layer in the composition, i have to create a little solid right under the hole and color it like i want?
Like Dave said, just put a white solid under your video layer.
You don't 'have to'. You can, if you want.
Supposing you have a character in a composition walking along a riverbank and talking. The character is a separate layer and you cut hole in where its mouth is located, but for some reason you don't want to see a part of the river or the sky inside its mouth... In this case you may want to fill the hole with a colour, whereas the rest things on a background, including the river and the sky, remain.
Adding a mask means you are cutting a hole in the layer. You are going to see what ever is underneath the layer through that hole. If you want it Green then put a green solid under the layer. If you are seeing black then you are seeing the default Composition Background Color.
Search Masking in the AE Search Help Field at the top right corner of the app and read up. There are some good examples in help.