I think these are the settings you are looking for ?
On the commandline, or in your mame.ini, enable hlsl (hlsl_enable), use video d3d, and set 'phosphor_life' to something (for example, '-phosphor_life 0.6500000,0.6500000,0.6500000'), so you would for example have a commandline looking somewhat like this :
You need to enable 'hlsl' before the 'phosphor persistence' (blue, green, red) options become available in the MAME internal menu. (Also, you need to have d3d video enabled [the default on Windows as far as I know] because hlsl only works with d3d and not with opengl/etc.). I don't know how to enable hlsl in MAMEUI; I can't find an MAMEUI interface option to enable hlsl. Perhaps someone else can tell you.