What color of Asha is the most realistic? What color is best?
Asha's are graded on the same color scale as natural diamonds, and thus no color is more realistic than another, because natural diamonds are available in the same exact colors.
Thus, an F color Asha is the same color as an F color natural diamond, and an H color diamond is the same color as an H color Asha.
As far as what color as best, market prices are higher for better color diamonds (D/E/F), so from a value perspective a colorless (D/E/F) Asha may be perceived as having a more valuable appearance. However, many customers also prefer lower color grades (G/H/I) as this is the color grade they could more easily afford if they bought a natural diamond (and thus an Asha in the same color grade is more realistic for them to pull off as a natural diamond).
If it helps, the consensus from our customers is that the E/F colors are 'icy' in appearance, and the G/H colors have a bit more steeliness to them. So that may sway your preference one way or the other.
In the end, it is important to note that the distinctions between color grades is quite fine - there is not a dramatic visual difference between an F and an H color. The same natural diamond may receive an E color grade from one lab, while another lab grades in an F, which illustrates how fine the differences can be. Mounting the stone further dilutes the distinction, which is why all diamonds must be graded in loose form.