Can the colour change in litmus paper be explained by conjugated systems? I have been trying to understand the colour change in litmus paper on a more detailed level than "the chemical identity changes, and therefore also physical properties". I have knowledge of the particle in a box model of quantum mechanics, and i would like to apply this to the conjugated system in the active indicator-component of litmus dye.
In other indicators I have looked at, you can clearly see that when the dye molecule dissociates or is protonated, there is change to the length of the conjugated system (number of alternating double bonds in a row), which explains the colour change. An example is given below for BTB (bromo thymol blue)


However, i can't find any explanation like this as to why the indicator dye in litmus paper (7-hydroxyphenoxazone) changes colour when protonated/deprotonated. From what I can see, the length of the conjugated system is the same in both versions of the molecule.


Edit: If someone knows of a model that more easily explains this colour change than conjugation, feel free to share that as well.
You have shown the structural change in bromothymol blue upon deprotonation.     

For the anion of 7-hydroxyphenoxazone, the following resonance structures involving a p-quinone imine are conceivable: 



The assumption that the $\pi$-system of the neutral species and the corresponding anion are identical is not justified.

A smaller system, in which a similar effect can be observed, is 4-nitrophenol. A solution of the phenol is colourless, whereas the solution of the phenolate is yellow ($\lambda_{\mathrm{max}}$ = 405 nm). 

Again, loss of a proton leads to tautomerism with contribution of a quinoid structure.