How to make a part change color?

Sorry for the title, it wouldn't allow my standard title. Me and my friend are making a game MMORPG game and we have been experiencing issues. For instance, one of our main problems is the interface. When I would do something on my script, my friend couldn't perform the same action on his side. For example, my double jump script only worked for me, and not him. My combat script only worked for me, not him. If anyone knows why this is happening, please tell me. Secondly, animations. I know this is a well-known problem on the studio, but if there is any way to share animations, and make them compatible with both players, please tell me. When I use his animations in a script, they don't play.

local anim = script.Animation
local play = hum:LoadAnimation(anim)
play:Play()
-- this code would not play for me, because it isnt my anim

I know this wouldnt work, its just an example.