[topic moved to Reader/Acrobat DC for Mobile forum]
Open the PDF in Acrobat Reader
From the bottom Tool set select Comment > Freehand
After making the freehand drawing click Save on the top right
Then tap & hold on that freehand drawing in the PDF and will get a contextual menu just above it in black bar.
The menu will have options: Delete, Thickness, Color, Opacity
From here you can change the color.