At first I wanted to just write: "A photograph slowly losing its colors."
But I think losing isn't the right verb here. What can I use instead of losing? (I want to keep the word colors.)
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As you've said you want to keep the word "colors", I presume you want a transitive verb to fit into the sentence "A photograph slowly ing its colors". If so, I would suggest the following:
To be honest, I think the sentence might be better rearranged, but I'm inferring from the way you worded the question that you don't want to do that.
Desaturate is commonly used to refer to the loss of color in photos and of color in general.
desaturation [diːˌsætʃəˈreɪʃən] n (Physics / General Physics)
Physics the addition of white light to a pure colour to produce a paler less saturated colour (TFD)