Pencil highlighter

I saw on the internet that Kaweco has a 5.6 mm highlighter lead, and I think it is just awesome. I like the idea of using a pencil highlighter, you do not get smudges, no bleed, and the pencil will not fade with time. But it is pretty hard to get the 5.6 mm leads, as they don't seem to be very popular. From the searches made almost the only place you can buy them is jetpens.com
They come either mixed, a yellow, a red and a green or a pack of 3 with the same color inside. At the moment of writing this post, the pack of 3 is $ 6.5 and a free shipping in USA for orders above $ 25
They look cool and vibrant but why not use a regular color lead instead of the "highlighter" ones?

I made a short test with the Koh I Noor 2 mm lead 6 color pack. All the colored leads are very good as a highlighter, as they are vibrant enough to be seen but not too intense that they obscure the writing beneath. The downside is the small diameter, and the fact the leads are not extra soft. Because of this, you will need a shading move of the pencil to highlight the row. So maybe a little extra work. I wonder how the 5.6 colored Koh I Noor leads work for this kind of stuff.
Bellow is the Kaweco highlighter (picture from jetpens.com) and to the right is the Koh I Noor regular color lead.

picture from jetpens.com