abstract Paint{T, N}

abstract AbstractColor{T, N} <: Paint{T, N}
abstract Color{T}            <: AbstractColor{T, 3}
abstract Transparent{C<:AbstractColor,T,N} <: Paint{T,N}

immutable RGB{T<:FloatingPoint} <: Color{T}
    r::T # Red [0,1]
    g::T # Green [0,1]
    b::T # Blue [0,1]
end

colortype{C<:AbstractColor    }(::Type{C})                  = C
colortype{C<:AbstractColor    }(::Type{Transparent{C}})     = C
colortype{C<:AbstractColor,T  }(::Type{Transparent{C,T}})   = C
colortype{C<:AbstractColor,T,N}(::Type{Transparent{C,T,N}}) = C

T = TypeVar(:T)
colortype(RGB{Float32})                     # yields RGB{Float32}
colortype(RGB)                              # yields RGB{T<:FloatingPoint}
colortype(RGB{T})                           # yields RGB{T}
P = Transparent{RGB{Float32},Float32,4}
colortype(P)                                # yields RGB{Float32}
P = Transparent{RGB{T},T,4}

julia> colortype(P)
ERROR: LoadError: MethodError: `colortype` has no method matching colortype(::Type{Transparent{RGB{T},T,4}})
 in include at ./boot.jl:254
 in include_from_node1 at ./loading.jl:264
while loading /tmp/color_trouble2.jl, in expression starting on line 25

C = TypeVar(:C,AbstractColor)
N = TypeVar(:N)

julia> typeintersect(P, Transparent{C,T,N})
Transparent{RGB{T},T,4}

julia> P <: Transparent{C,T,N}
true