Assume an overriding aim that the four forces of physics are unifiable. Therefore all the forces must have a lot in common as the original or ancestral unified force must be a coherent and fully-functioning composite of all other forces.
QCD is colour-based, with an SU(3) group structure while QED is colourless [or rather is a neutral mix of colours] with a group stucture embedded within an electroweak structure. The group structure of SU(3) allows QCD-colour to behave like light in that one colour contains others within it. Specifically, adding Red + Green + Blue gives white. Re-using this suggests QED white charge is an amalgam of QCD colour charges.
It is clear that a red up quark has a positive charge while a red down quark has a negative charge. This is a major stumbling block for accepting that, at my preon level, QCD colour (e.g. red) has QED electric negative charge while QCD anticolour has QED positive electrical charge. QED particles are colour neutral having either white (negative) or black (positive charge). In a preon model, some aggregates of preons can be net white or net black and adding those aggregates into the quark can change the overall sign of the electric charge at quark-level.
The Weak force is harder to cope with in detail as it is affected by the higgs field, but basically weak isospin is an electric vacuum field and fits into a total composite unified force.
A graviton, acting on a mass charge with spin 2, has two incorrect features which need to be corrected. The first is that mass, as a charge, is not present in QCD or QED or Weak. And mass does not feature in my model as either a charge or a fundamental quality of a preon. Spin 2 seems to me only to be required to cope with having a plus sign always associated with mass. A spin 1 boson would repel two masses, so spin 2 is required to attract two positive mass charges. Eschewing spin 2 goes along with eschewing the mass charge. Using a spin 1 graviton acting on colour charges unifies gravitation with the other forces and allows the four forces to be integrated into a single composite force.
Gravitational colour is weaker than QCD colour so imagine an optical cable with very many strands representing QCD. Gravitation is enforced through just a few strands which have spit off from the main bundle. The force of QCD is approximately one thousand million million million million million million times (that is 10^39 times) stronger) than the gravitational force so there needs to be that many more fibres in the QCD ‘cable’ than in the gravity ‘cable’.
In my model the photon, Z and gluon are three family members. Gravity, in my model, can be enforced by these same three bosons, so long as all elementary particles contain gravitational colour/anticolour, similar in colour format to the QCD colour contained in the gluon, but a thousand million million million million million million times weaker.
The three bosons can provide the same suite of features as QED and QCD but on a very much larger scale of distance (10^39 times larger). The QCD-like feature providing the generally attractive quality of gravity up to 60 million light years. The QED-like aspect causing dark energy repulsion at an even greater distance.
For more, see my vixra paper at http://vixra.org/abs/1709.0021
Hexark and Preon Model #8 and the Unification of Forces: a Summary
This paper summarises a model for building all elementary particles of the Standard Model plus the higgs, dark matter, dark energy and gravitons, out of preons and sub-preons. The preons are themselves built from string-like hexarks each with chiral values for the fundamental properties of elementary particles. The four forces are shown to be unified by hexarks being string-like objects comprising a compactified multiverse-like structure of at least 10^39 strands of string-like 4D space and time blocks (septarks). Despite the individual forces seeming very different from each other, they all derive from the same colour strands, either as net colour braids (QCD and attractive gravity) or as net neutral-colour braids/strands (electric charge, weak isospin and dark energy, or repulsive gravity). Different strength forces have different numbers of braids in them but QCD-colour is qualitatively, but not quantitatively, the same as gravitational colour while electric charge, weak isospin and dark energy are all qualitatively the same neutral-colour mix, but not quantitatively the same.