If you're using the default skin (Kerni-HD1), then edit the file /usr/share/enigma2/Kerni-HD1/skin.xml
Find the <subtitle> section and locate this part:

<sub name="Subtitle_TTX" font="Regular;30" shadowColor="#40101010" shadowOffset="3,3" />

and add a color definition:

<sub name="Subtitle_TTX" font="Regular;30" foregroundColor="#FFFFFF" shadowColor="#40101010" shadowOffset="3,3" />

(notice the forgroundColor bit is being set to FFFFFF, or 'white' to us humans)

Now, you need to restart Enigma GUI:
Go to Menu -> Setup -> System -> Skin
Press OK when highlighting your active skin and choose Yes to restart the GUI.

I assume this would also work for a different skin, provided you edit it's associated skin.xml file.

"Regular;30 CAN BE CHANGED TO INCREASE OR DECREASE SUBTITLE SIZE !