Changing colouring through a line of text

[uzibalqa]

I am doing a bash function that takes a multiline string and prints each line.  
The first part of each line uses a blue foreground, whereas the rest of each 
line uses a white foreground.

I want to have the user define a column number from which the foreground colour 
on each line changes to white. 

I need help to do this.

The input string would be 

str='
 marinex-dgrul    Digit
 marinex-ltrul    Letter
 marinex-nmrul    Numeric'

Then one would call

marinex-ndic 18 "$str"

The plan is to have the beginning of each line being blue, whilst from column 
number 18 the colour changes to white.

marinex-ndic ()
 {
  ## Show coloured  and label

  local -r rst="$( tput sgr0 )"  # Default Graphic Rendition
  local -r blu="$( tput bold; tput setaf 39 )"  # BLUE
  local -r wht="$( tput bold; tput setaf 15 )"  # WHITE

  printf '%s\n' "$@"  \
    | while IFS="" read -r vl; do
        printf '%s%s%s%s%s\n' "${blu}" "$vl" "$wht" "$vl" "${rst}"
      done
 }