How to blend 2 same colors, light and dark, in gimp 2.10

[fenpeppertree]

How can I get a hazy-like blend with a light and dark color of the same color, so that the color is not a static, plain, solid color in Gimp 2.10? 

I have tried the smudge tool, but it erases down to the alpha channel background in places, which just makes it very difficult, not to mention annoying and unsatisfactory.