[Konstantinos]

Keep in mind that the function have already a parameter "color" so you won't have to color embedding at the beginning of the string. Also the alpha transparency don't work with client messages.

pawn Код:

if (InMM[playerid]) return SendClientMessage(playerid, 0xFFFF00FF, "ERROR: You are already in a DM arena! Type /leave to exit!");

If you want to have "ERROR: " in yellow and the rest with white or any other color, then yes - use color embedding: https://sampwiki.blast.hk/wiki/Colour_Embedding