Here a step-by-step tutorial:

Step 0: $ pkg install neofetch nano

Step 1: $ nano /data/data/com.termux/files/usr/bin/neofetch

Step 2: go to about line 5490

Step 3: replace the line that starts with set_colors (the one under the line that says “Android”) with set_colors 6 13 15 13 6

Step 4: set the colour tags (the thing that says ${c1}) acordingly (like in the picture):