4.123 A property is internal if it is unthinkable that its object should not possess it.
(This blue color and that stand in the internal relation of lighter and darker eo ipso. It is unthinkable that this pair of objects not stand in this relation.)
(Here the shifting use of the word “object” corresponds to the shifting use of the words “property” and “relation.”)
Marie McGinn (p. 182): “In the later philosophy, it is clear that Wittgenstein thinks that the colour-wheel is itself a part of the symbolism, in the sense that the ordered colour samples of the colour-wheel constitute an instrument of our language, by means of which the logical order of our colour concepts is presented. However, it is not clear that he held this view at the time of writing the Tractatus, where he seems to suggest that the logical order of colour-space will be revealed through the logical analysis of colour terms (see TLP 6.3751).” McGinn also discusses Remarks on Colour p. 34 in connection with this.
So, this is what is special about facial features: they have a kind of essentiality. The reference to shifting uses of words here might alert us to the possibilities that LW’s example does not really tell us anything about what he has been talking about till now (objects, etc. in a different sense) and that nothing really can be identified as what he has been talking about till now (objects, etc.). On the other hand, isn’t there a relation of darker/lighter internal to a pair of shades of blue? And a similar essential relation between earlier/later, left/right, and so on? The very necessity involved (seemingly) here makes them, perhaps, not relations proper (matters of fact), but don’t they indeed seem to be relations of a kind? Isn’t one essentially less than three? Perhaps LW wants us to see such apparent metaphysical truths as misunderstood points of logic, definition, stipulation, convention, or grammar.