Assuming you want a new List of CustomClass use string.join and groupby
var list = new List<CustomClass>
{
new CustomClass {Group = "Car", Color = "White"},
new CustomClass {Group = "Motocycle", Color = "Blue"},
new CustomClass {Group = "Car", Color = "Black"},
};
var results = list.GroupBy(c => c.Group).Select(g => new CustomClass()
{
Group = g.Key,
Color = String.Join(", ", g.Select(c=>c.Color))
}).ToList();