The Sun is not a point source, the photometric observational techniques that are utilised for observing other stars cannot be utilised for the Sun, meaning that it is difficult to derive its colours accurately for astronomical work from direct measurements in different passbands. Until recently, most of the work concerning the subject has been based on indirect methods.

The solar twins are the best choices because they are the stars that are ideally the same as the Sun in all parameters, and also, their colours are highly similar to those of the Sun. From the 60 articles on the Sun and Sun-like stars in the literature from 1964 until today, the solar colour indices in the optic (U, B, V, R, and I filters) and infrared regions (J, H, K, L and M filters) have been
estimated.

Accordingly, the solar colour indices are B-V = 0.6457, U-B = 0.1463, etc. The distribution of the colour indices (B-V, U-B, etc.) for the G2V spectral type shows that these are good criteria for the solar twin stars and are in the ranges of 0.606 (±0,006) ≤ B-V ≤ 0.688 (±0,004), 0.054 (±0,014) ≤ U-B ≤ 0.228 (±0,012), etc. Based on these colour indices, these Sun-like stars have been listed in the ASAS and are considered solar analog or solar twin stars.