Looked everywhere but can't find a mod that simply let you choose the color of the civ you're playing.

Any idea or does such a simple mod don't exist ?

I don't know if there's a mod for it or not, but changing your civ's colour is a simple change through XML.

Say you wanted to change the civ colour for the Ottoman civ from Dark Green to Red, then make this change in Civ4CivilizationInfos - Program Files/Fixaris Games/Sid Meier's Civilization 4/Beyond the Sword/Assets/XML/Civilizations/Civ4CivilizationInfos.

<CivilizationInfo>
<Type>CIVILIZATION_OTTOMAN</Type>
<Description>TXT_KEY_CIV_OTTOMAN_DESC</Description>
<ShortDescription>TXT_KEY_CIV_OTTOMAN_SHORT_DESC</ShortDescription>
<Adjective>TXT_KEY_CIV_OTTOMAN_ADJECTIVE</Adjective>
<Civilopedia>TXT_KEY_CHANGED_CIV_OTTOMAN_PEDIA</Civilopedia>
<DefaultPlayerColor>PLAYERCOLOR_DARK_GREEN</DefaultPlayerColor>

to

<CivilizationInfo>
<Type>CIVILIZATION_OTTOMAN</Type>
<Description>TXT_KEY_CIV_OTTOMAN_DESC</Description>
<ShortDescription>TXT_KEY_CIV_OTTOMAN_SHORT_DESC</ShortDescription>
<Adjective>TXT_KEY_CIV_OTTOMAN_ADJECTIVE</Adjective>
<Civilopedia>TXT_KEY_CHANGED_CIV_OTTOMAN_PEDIA</Civilopedia>
<DefaultPlayerColor>PLAYERCOLOR_RED</DefaultPlayerColor>

Then save, and load the game as normal.

Thanks for the tip

It's a bit tedious though to do this at every game, i was hoping for a small mod/plugin.

But i think you're right, this is the only way to choose your civ color