Facultativ Species of the genus Rhodopseudomonas constitute the majority of phototrophic purple non-sulfur bacteria,and are characterized as rod-shaped, motile cells that show polar growth and asymmetrical division or budding as a mode of reproductione photo-organotrophic, purple, non-sulfur bacterium. Several red or red–brown colonies of different sizes and textures appear after five to seven days of incubation under anaerobic/light conditions. There may be some problem in medium pH, nutrient composition , or having a different strain. The most dominant red colony was selected for purification. Several representatives of single red colonies were individually transferred into screw cap tubes filled with nutrient broth medium and incubated under the light at room temperature for enrichment. Be ensure and check the purity of cell culture, the streak dilution platesshould prepared from each tube containing the originally isolated colony on nutrient agar medium, and checked for purity. The gram-negative rod-shaped bacterium,  produce a dark red culture under phototrophic conditions, reproduce by budding and form a lamellar intracytoplasmic membrane (ICM) system parallel to cytoplasmic membrane, which contain  bacteriochlorophyll a and carotenoids.

Exposing to aerobic dark or light conditions may produce colourless R. palustris colonies

There are likely several different reasons that could possibly explain why you did not see pigmented colonies, but the most likely reason is as follows:

It is that there were no colonies of R. palustris present at all, or only what Bergey's manual gently references as "anomalous growth" (likely colorless as well) due to the fact that you may have used a general-purpose nutrient agar as a medium, rather than one of the four or five very specialized -- and much more rare, in terms of ready availability -- nutrient agars that are needed to successfully culture purple non-sulfur bacteria such as Rhodopseudomonas palustris.

Another reason for the apparent failure to culture this PNSB may be that your original culture stock, that is, the culture with which you inoculated the plates, may not have been a pure R. palustris culture. In fact, given your employment setting (I am familiar with your company, and with its products, and have provided consulting to some of your predecessors in past years), I suspect strongly that the culture stock that you used may well have been a mixed microbial consortium containing a large quantity of lactic acid bacteria (LAB) and only small quantities of PNSB, and also exhibiting a very low pH, in the range of 3.0 (due to the acids produced by the LAB).

In such a case, unless you are using extremely specialized PNSB-specific culture media that will discourage growth of the LAB and concomitantly neutralize the low PH of the inoculant culture and raise the pH of local micro-environment on the culture plate to above 6.9, R. palustris will either not grow at all, or it will grow so slowly that it will never produce any observable pigmented colonies.

If my latter conjecture as to the mixed microbial consortia/acidic nature of the inoculant culture was true, then you may wish to entirely give up on trying to use conventional agar plate culture methods (which, in any case, even with the aid of highly specialized media, can successfully culture only about 5% of known bacteria; this is known as the Great Plate Anomaly) to detect the presence of R. palustris, and instead you may wish to send your culture sample to a qualified lab for targeted microbial metagenomic analysis; the best-fit method for your needs would likely be NGS (next generation sequencing) targeted bacterial 16S sequencing analysis. This modern genetic testing method, which does not involve any culturing at all, will render reports that will disclose all bacteria present in the original culture stock, and will indicate the relative abundance of each species as well.