If you use windows... just open it with "Paint" application. Then open the other official color via another tool called "snipping" (windows-only). This last tool allows you to grab (print screen) a portion of your screen, where you can grab the official color from the hive.io/assets page. Then paste it on a corner or "far" away place on the original avatar picture. Then you have a little icon on "Paint" that looks like a Pipette. It basically allows you to copy the color of your drawing into the "drawing" color. Then you can use the "bucket" icon to cover the blue part of your avatar with that color. Then, after all good, remove the paste of the "snipping" tool you added to your main drawing... and save your picture once with the right dimensions (I am guessing you will need to enlarge the canvas to paste the snipping part).

If this is probably very confusing... I will try to make a post later today and explain step by step =) or if you can't wait for it, I can do it for you in a few seconds... just need the link for the post where you got that avatar and I can change it for you.