A blend mode might work.
Place the medal layer on the bottom. (As a substitute, the example to follow uses a character from the Street Fighter Alpha series.)
Fill a top layer with a color representing bronze, silver or gold.
In layer properties, change the top layer’s blend mode to Color.
Hold down Ctrl or Cmd and click on the layer’s name in the timeline to select its cel contents.
Invert the selection.
Delete the excess top layer fill.
imo, a gradient map would give you more control over the results. Gradient maps are not a built-in Aseprite feature, however. You’d have to use a Lua script or another graphics editor, like Krita or Gimp.
Compare the blend mode approach for gold
with a gradient map approach
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