Like the RGB version, with an additional Alpha value (opacity).
Warning : an alpha value of 0 means a 0% opacity color, thus 100% transparent.
color r,g,b,a
r red g green b blue a opacity (0-100)
Source for 10 to 100 step 15 as ?alpha color 100, 100, 0, ?alpha trace text "TEST" center 0, -?alpha nextExecution TEST TEST TEST TEST TEST TEST TEST