Colorized Layer Reverts to Greyscale if I Add a Layer Above it

[TMORT]

I have some satellite imagery of the same area bur from different times. I want to show where development has occurred.

The satellite images are all the the same extent but come from different sources and look a lot different from one another. While the changes in developed areas is visible it gets lost in all the other differences in the images.


I thought it might be best to use edge detect with the magic wand to select the developed areas in another layer  and then turn it to just black and white, then change the colors of that area to some other color. I also thought because the satellite images were quite variable in how they looked, some were nice and full color others were more black and white and some it looks like some tiles were color and others more grayscale.

I could do that. I later thought I'd colorize the base grayscale layer to green. It sort of matches what I'm trying to show and gets lost even in the full color images when I go to grayscale. 

I find though that when I overlay the layer with just the developed areas with an alpha background that the bottom colorized layer loses it colorization. I read something about turning the alpha lock on so I did that for the base layer and also the top layer and that didn't change anything. It went from colorized back to grayscale.

Does anyone know why it is doing this and how to keep if from doing this.

For that matter, I described what I'm trying to do because I have to think this sort of thing has been done many times by others. Maybe there is another way that is easier or maybe that just works.

I have used GIMP on and off for quite a few years now, but never really for this sort of thing. I'm also using v3.06 for Linux. I think it is a flatpack build as well for what its worth.

[Tas_mania]

When I have made map overlays, I prefer to 'scale' smaller layers up or down a base layer. You can also adjust width and height independently if you need to. Not much will get lost probably   Your main friend here is the opacity slider which can be set differently for any layer above a base layer.

[TMORT]

all the layers in this case are the same size to begin with and line up.  The top layer (there will be more) also has an alpha background and just some parts which can be opaque or semi opaque. So the areas that are opaque or semi opaque do obscure what it beneath them. That is what I want.

It is the area under the alpha background that is the issue. It is clearly visible, but the colorization is lost, I just see it as greyscale.

I don't understand why it behaves this way.  I'm using all png layers. Maybe some other image format maybe?

[rich2005]

Not able to reproduce that so far.  I assume that greyscale = desturated and the image is RGB
Things to look for:
In Gimp 3 colorize is a layer effect (fx) If you have that fx against the layer try again with the merge filter box ticked.
Used to be that some higher bit depths 16 / 32 bit affected transparency. I think all that is fixed now but check.