In AI 26 using Mac How can I add to a line ?

l,

There are always (many) different ways of doing things with the help of Illy, and she is always eager to help, even in silly ways if you wish.

Another one is to use a single line with the total visible length as follows, assuming 1.25 " magenta at both ends of a black middle part (you can also use it on a curved path when you know the length(s)):

3.1): Create a (horizontal) line (maybe with the Line Segment Tool) with the total length (the length of the black middle part + 2.5") and give it the end magenta colour;

3.2) In the Appearance panel Add New Stroke and give it the black middle colour, then in the Stroke panel tick Dashed Line and insert the following values: 1st Dash = 0, 1st Gap = 1.25", 2nd Dash = [The length of the black middle part], 2nd Gap = 1.25".

If desired, you can adjust the lengths.

For a horszontal line with symmetrical ends you can also:

4.1)): Create a (horizontal) line (maybe with the Line Segment Tool) with the total length (the length of the black middle part + 2.5") and give it the end magenta colour;

4.2) In the Appearance panel Add New Stroke and give it the black middle colour, then Effect>Convert to Shape>Rectangle with Relative ticked and Extra Width = -1.25 and Extra Height = 0.

Using 2 lines, you can:

5.1) Create the full length magenta line and copy it on top of itself;

5.2) Change the colour (of the copy) to black and reduce the length, adjust the horizontal position if the ends are to be asymmetrical.

There are two possibilities:
1. you create three lines and colour the parts red, black and magenta.
2. you use a gradient with sharp transitions, as I explain in the following:
(A) Choose a gradient for the outline colour. In the gradient tool you need two colour points each for red and magenta. In order to have a sharp transition, the colour points black and magenta have to be (almost) in the same position, i.e. 90% and 90.01%.
(B) The second point of red is in the (almost) same position as the black point that covers it.

 To extend the line by 1.5 mm on both sides, you can simply enter the following in the Transform palette:
(D) Select the centre as the anchor point.
(E) Add 2x 1.5 mm to the length of the line, i.e. 3 mm. Illustrator works like a small calculator and calculates "30 mm + 3" to 33 mm after pressing the Tablulator key or Enter.