**Background:** The assessment of visual function in visually impaired patients requires evaluation of multiple parameters including color vision, contrast sensitivity, and photostress recovery. Color vision abnormalities are common in various ocular diseases and may serve as early indicators of disease progression. This study aimed to investigate correlations between color vision tests (K-colour, HRR, Ishihara) and visual acuity, contrast sensitivity, and photostress recovery time in visually impaired patients with different etiologies.
**Methods:** This cross-sectional study enrolled 133 patients (133 eyes) prospectively from an outpatient unit. The sample included 76 males (57.1%, mean age 68.0 years, SD=13.2) and 57 females (42.9%, mean age 68.1 years, SD=15.2). Causes of visual impairment were age-related macular degeneration (AMD, n=52, 39.1%), diabetic retinopathy (DR, n=27, 20.3%), and other causes (n=54, 40.6%). Mean best-corrected visual acuity (BCVA) was 0.70 logMAR (SD=0.24). Visual impairment was categorized per WHO classification: mild (worse than 6/12 to 6/18, n=37) and moderate (worse than 6/18 to 6/60, n=96). Exclusion criteria included high refractive error, corneal opacity, acute uveitis, acute glaucoma, significant lenticular changes, pharmacological treatments affecting vision, and visual impairment from multiple etiologies. Color vision was assessed monocularly using the HRR test (6 screening plates), Ishihara test (38 plates, abnormal if ≥4 errors in first 21 plates), and the K-colour test (11 plates with decreasing brightness for red, green, and blue). Contrast sensitivity was measured using the Pelli-Robson chart at 1.67 m (1.5 c/d) and 3.34 m (3.0 c/d). Photostress recovery time (PSRT) was measured using a smartphone-based application with a bleaching light at ~5 cm for up to 10 seconds, recording time to read three successive letters of the BCVA line. Statistical analysis used Mann-Whitney U test for group comparisons and Spearman's correlation coefficients, with significance set at P≤0.05.
**Key Results:** Patients with mild visual impairment had significantly higher scores than those with moderate impairment in K-colour tests (Red: median 3 vs 1, P<0.001; Green: 4 vs 2, P<0.001; Blue: 5 vs 4, P=0.002), HRR (3.5 vs 0.5, P<0.001), Ishihara (30.5 vs 4.5, P<0.001), contrast sensitivity at 3 c/d (1.35 vs 0.90, P<0.001) and 1.5 c/d (1.35 vs 1.05, P<0.001). PSRT showed no significant difference between groups (30 vs 28 seconds, P=0.907). In AMD patients (n=52), significant positive correlations were found between contrast sensitivity at 1.5 c/d and all color tests (K-colour Red r=0.55, Green r=0.55, Blue r=0.48; HRR r=0.57; Ishihara r=0.71). Contrast sensitivity at 3.0 c/d showed moderate positive correlations with K-colour tests (Red r=0.59, Green r=0.48, Blue r=0.42) and HRR (r=0.54), and strong correlation with Ishihara (r=0.77). logMAR VA showed weak negative correlations with K-colour Red (r=-0.31) and Blue (r=-0.29), HRR (r=-0.38), and moderate negative correlation with Ishihara (r=-0.55). PSRT showed moderate negative correlations with K-colour Green (r=-0.34), Blue (r=-0.40), and HRR (r=-0.33). In DR patients (n=27), contrast sensitivity at 1.5 c/d showed weak positive correlation with HRR (r=0.77), moderate with K-colour Red (r=0.60), Blue (r=0.53), and Ishihara (r=0.65), and strong with K-colour Green (r=0.55). Contrast sensitivity at 3.0 c/d showed moderate positive correlations with K-colour Red (r=0.63), Green (r=0.55), HRR (r=0.65), and Ishihara (r=0.49). PSRT showed moderate positive correlations with HRR (r=0.56) and Ishihara (r=0.66). No significant correlations were found between logMAR VA and color tests in DR patients.
**Clinical Implications:** This study demonstrates that color vision tests correlate significantly with other measures of visual function in visually impaired patients, particularly in AMD and DR. The K-colour test, a smartphone-based digital test, shows promise as a practical clinical tool for assessing color vision in acquired deficiencies. The findings support the inclusion of color vision testing alongside visual acuity and contrast sensitivity for a more comprehensive functional vision assessment. The lack of significant difference in PSRT between mild and moderate impairment groups suggests that photostress recovery may be less sensitive to severity of visual loss in this population. Limitations include the absence of anomaloscope comparison (gold standard for color vision defects) and lack of standardization in PSRT testing. Further studies are needed to validate these correlations and establish clinical utility.