I’d recommend you read
and edit your question accordingly to turn it into an MWE so that people can help you more easily.
Without knowin what red or j is, I’d say your code shouldn’t be valid Julia:
julia> x = rand(3, 2);
julia> sin.(x[:, 2]) = sin.(x[:, 2])
ERROR: syntax: invalid syntax "sin.(x[:, 2]) = ..." around REPL[34]:1
The left hand side of = should be something you can assign to, which red.(j[:, i]) most likely isn’t. Maybe you meant
j[:, i] .= red.(j[:, i])
but again it’s hard to tell without more information.