Highly conjugated compounds and the effects of substiuents
Yes, your intuition is correct. 

Among others properties, systems with pi-electrons delocalisation are actually chromophore whose UV-Vis absorption spectrum is mainly dictated by the HOMO-LUMO transition.

An extension of the delocalised system results in a narrowing of the gap and thus shifts the spectrum to red. The effect is dramatic when extending, e.g., the length of a polyene chain.

However, although with different impact, the same is true if a parent system is substituted with group leading to additional resonance, both because of electron donating or withdrawing effects.

This is fundamentally no different from considering a C=C double bond, which is itself a chromophore, and functionalize it with the various auxochromes that are commonly listed when introducing UV-Vis electronic absorption spectroscopy.

An auxochrome as listed won't necessarily delocalise over the conjugated system in complicated structure, or it can marginally do so. Thus, don't assume that if there is a methoxy - just to mention an auxochrome - somewhere in the molecule or name, the spectrum of the latter is necessarily red shifted as compared to the parent molecule lacking that auxochrome.

If the auxochrome can indeed partecipate in delocalisation, then its effect is bathochromic (red shift) and generally hyperchromic (higher absorbivity).

Rules for predicting the wavelength in the absorption spectrum of conjugated systems do exist, though they fail quite early with increasing the conjugation length.

Consider that the conjugation length of a system is better described by an effective conjugation length, as for the system is susceptible of various effects, first of all departure from planarity due to hindrance. The same is true for the effect of the substituent. If you think of a very long polyene, an end substitution doesn't necessarily extend its effect all along the formally conjugated system.

Similarly the concomitant presence of electron withdrawing and donating groups can have negligible or no effect, as their influences can somehow cancel each others. 

Again, is very important to consider that the effective delocalisation is not only influenced by the size of the system, but by an ensemble of parameters, such as the already mentioned steryc hindrance, structural motifs that might pin electrons (think of a phenylene in the middle of an otherwise polyene chain), solvent, etc.

With a bit of caution, you can certainly take that substitution of a conjugated system with groups having mesomeric effects results in absorption maximum at longer wavelength as well as in a higher absorptivity.