To increase the number of red (H3O+) particles, you can adjust the settings of the simulation to increase the concentration of an acid being added to the solution. Simultaneously, to increase the blue (A-) particles, you can introduce a weak acid where the equilibrium favors the dissociation of the acid into H3O+ and A-. By increasing the concentration of acid, the equilibrium will shift according to Le Chatelier's principle, resulting in more H3O+ ions being produced and consequently more A- ions from the dissociation.
This adjustment will lead to a decrease in pH, as pH is inversely related to the concentration of H3O+ ions—the more H3O+ present, the lower the pH. A higher concentration of A- may also indicate an increase in the degree of dissociation of the acid, further confirming the shift towards more acidic conditions in the solution.