distances on colorant

```julia
julia> Euclidean()(rand(Gray), rand(Gray))
ERROR: MethodError: no method matching (::Euclidean)(::Gray{Float64}, ::Gray{Float64})
Stacktrace:
 [1] top-level scope
   @ REPL[22]:1
 [2] eval
   @ ./boot.jl:360 [inlined]

julia> Euclidean()(rand(), rand())
0.15281823883266332
```

Will fixed it later.