When a culture that has comma for decimal separator is used, the ColorPicker formats the rgba() CSS rule as rgba(255,255,255,0,6) which is incorrect. The alpha value should always have a dot as a decimal separator.

<TelerikColorPicker @bind-Value="@value">

@code {
    public string Value { get; set; } = "#282f89";
}

Set the culture of the application so that the default decimal separator is a comma (el-GR).

// in Program.cs file
CultureInfo.DefaultThreadCurrentCulture = new CultureInfo("el-GR");
CultureInfo.DefaultThreadCurrentUICulture = new CultureInfo("el-GR");

The rgba() rule is formatted as rgba(255, 255, 255, 0,6) but should be formatted as rgba(255, 255, 255, 0.6).