It is a very light yellow. People draw it as white because it's easier. Just like it's easier using red then magenta. Then again, people have no problem drawing Rainbow Dash with the correct eyes, just cause Vinyl is a background character doesn't mean she should get treated any differently.
"While DJ Pon-3's coat appears white in her first scene, a layout artist on the show noted that the character model's coat color is actually a very pale yellow, specifically the 24-bit hex triplet #FEFDE7. This is more clearly seen in the episode, "A Canterlot Wedding - Part 2". Her coat color in the DJ Pinkie Pie game is the same, while her Playful Ponies toy is white. Her eye color is also revealed in A Canterlot Wedding - Part 2 to be Moderate cerise, the same as that of Rainbow Dash."