Hi, and thanks for your query on eHealth forum!
There are various causes for redness at the tip of the penis. Exact diagnosis is difficult without examination. However here is a list of diseases that can cause redness at the tip of penis: genital herpes, infection under the foreskin of uncircumcised men (balanitis), inflammation of the prostate gland (prostatitis), injury, cancer of the penis, Peyronie's disease, pimples, Reiter syndrome, sickle cell anemia, syphilis, and urethritis caused by chlamydia or gonorrhea.
It is possible to get false positive or false negative results with the standard tests for Chlamydia. Hence it would be wise to repeat the tests. Else you can opt for specific tests for Chlamydia which include Nucleic acid amplification tests (NAAT, to find the genetic material of Chlamydia), Nucleic acid hybridization tests (DNA probe test, Enzyme-linked immunosorbent assay (ELISA, EIA), Direct fluorescent antibody test (DFA) or chlamydia culture.
Since a confirmed diagnosis cannot be given on net please discuss with your urologist. Hope this helps. Take care!