I try to make a shadow over the source image. How to make it red in the correct way? I can not add my image to this post...

Couple of things:

You can use places like imgur to place your picture there and just use the Img or thumb= tags to display it here in the forum.

As for your issue, I'm not sure if I understand it, thus sorry if I'm getting it wrong, If I want to paint a shadow on an image that is already done (like photograph), I would place a new layer on top of it (transparent by default) then will reduce its opacity to let me see through, (this would not remove literally its opacity, it just makes it look transparent till you move the opacity slider) then just paint over with desired color.

If the shadow is already painted on an image and want to change color, depending on the detail and nature of the image, there would be many ways of changing colors, either manually or with filters and selection tools.

I think if you can tell us what exactly you trying to achieve we may be able to give you better suggestions.