For dpi, with last beta http://beta.plantuml.net/plantuml.jar you can have
@startmath
dpi 300
color red
f(t)=(a_0)/2 + sum_(n=1)^ooa_ncos((npit)/L)+sum_(n=1)^oo b_n\ sin((npit)/L)
@endmath
About having several colors, the issue is that each color will generate an independant image and it will difficult to align them.
Maybe we could allow:
@startmath
dpi 300
color red
f(t)
newline
color black
=(a_0)/2 + sum_(n=1)^ooa_ncos((npit)/L)+sum_(n=1)^oo b_n\ sin((npit)/L)
@endmath
Do you think it's worth trying ?