Allah U did not make the colour a condition on them in the beginning of the affair however they wanted to be difficult and made things difficult onto themselves so Allah U gave them conditions and traits that were found only in a specific cow.

﴾فَاقِعٌ لَّوْنُهَا﴿ “bright coloured” , that is of very bright yellow. And if they could find such a cow and slaughter it, the affair would have ended there.

But they came back asking: