You should use the fontRendererObj directly:
mc.fontRendererObj.drawString("fade string", sr.getScaledWidth() / 2 - mc.fontRendererObj.getStringWidth("fade string") / 2 , sr.getScaledHeight() - 97, color);
And don't forget to add the alpha:
int color += (alpha << 24)
That's how you get the transparency. For the fade-out you still need to setup a timer or use the partialTicks like in the GuiIngame.
I totally forgot about the subtitles and overlay messages from Minecraft!
In the GuiIngame class by line 241. Here is handled how to fade out and display a overlay message for a specific time. (When a record is played. In that fancy, colored way)
But like Matryoshika already said. Here you have to know how to use OpenGL.