Why doesn't throw_colorerror(and its caller checkval) use the color types directly?

Of course, it is just "for example", so it is OK...

julia> ARGB{N0f8}(100,150,200,250) # not RGBA
ERROR: ArgumentError: (100, 150, 200, 250) are integers in the range 0-255, but integer inputs are encoded with the N0f8
  type, an 8-bit type representing 256 discrete values between 0 and 1.
  Consider dividing your input values by 255, for example: RGBA{N0f8}(100/255,150/255,200/255,250/255)
  See the READMEs for FixedPointNumbers and ColorTypes for more information.

cf. #183

Anyway, in order to handle types like ACMYK{N0f8}, we need to add a 5-arg checkval.
Since Julia doesn't seem to fully do "horizontal" SIMD optimizations, I think there is room for optimizations.

Why doesn't throw_colorerror(and its caller checkval) use the color types directly?

Clearly it should!

IMO, Colors.jl, Images.jl and so on still need some time to properly handle the CMYK colors, so we should not add CMYK to ColorTypes for now.

I don't quite get the reason behind, could you please give some further explanation on this? Do you mean adding CMYK to ColorTypes doesn't make it generally a valid input to functions in Colors and Images because it has 4 components?

In the first place, Colors.jl does not have "practical" conversion methods between CMYK space and other color spaces. I think that CMYK conversion which is not based on color profiles is just a toy. 😄

Do you mean adding CMYK to ColorTypes doesn't make it generally a valid input to functions in Colors and Images because it has 4 components?

At least I don't think that has been fully tested. It is quite possible that there are codes other than the above which interpret the 4-component colors as RGBA or Transparent3 colors. Moreover, TransparentCMYK is a 5-component color.

In order to run such tests, it is important that ColorTypes.jl handles 2-/4-/5-component colors correctly first.

Tests for show.

using ColorTypes, FixedPointNumbers

# dummy type for testing 2-component color
struct AnaglyphColor{T} <: Color{T,2} # not `TransparentGray`
    left::T; right::T
end
# dummy type for testing 4-component color
struct CMYK{T} <: Color{T,4} # not `Transparent3`
    c::T; m::T; y::T; k::T
end
# dummy type for testing 5-component color
struct ACMYK{T} <: AlphaColor{CMYK{T},T,5}
    alpha::T; c::T; m::T; y::T; k::T
    ACMYK{T}(c, m, y, k, alpha=1) where T = new{T}(alpha, c, m, y, k)
end

iob = IOBuffer()

julia> show(iob, AnaglyphColor{Float32}(0.4, 0.2));

julia> String(take!(iob))
"AnaglyphColor{Float32}(0.4f0,1.0f0)"

julia> ans == "AnaglyphColor{Float32}(0.4f0,0.2f0)"
false

julia> show(iob, CMYK{Float64}(0.1, 0.2, 0.3, 0.4));

julia> String(take!(iob))
"CMYK{Float64}(0.1,0.2,0.3,1.0)"

julia> ans == "CMYK{Float64}(0.1,0.2,0.3,0.4)"
false

julia> show(iob, ACMYK{N0f8}(0.2, 0.4, 0.6, 0.8));
ERROR: MethodError: no method matching show_normed(::Base.GenericIOBuffer{Array{UInt8,1}}, ::ACMYK{Normed{UInt8,8}})

julia> String(take!(iob))
""

julia> ans == "ACMYK{N0f8}(0.2,0.4,0.6,0.8,1.0)"
false

This is off-topic but... what is this!? 😱

Although there are some problems with promotion rules etc., the original goal was almost achieved.