Hue-saturation pad is non-linear in HLS mode. the behavior is different when mowing the color circle and using the pad.

Add input rules and character count limit to the text box: 

 

1. There is no input mask, although the expected pattern is AARRGGBB.
2. Arbitrary and invalid characters can be input (#, +, %, Z, ...).
3. Invalid input is parsed to FF000000, but there is no indication that the input is wrong.
4. Shorter valid sequences, e.g. AB, are parsed, but to FFAB0000, instead of AB000000, which seems unintuitive.
5. Longer sequences exceeding the pattern can be input, although invalid.
6. Entering longer sequences leads to the expansion of the input box even beyond the bounds of the control 

Add an eyedropper tool like in Blend.

Users sometimes need to add additional color modes like Pantone, RGB 0-1. They need both adding the mode in the Combo and configure the Sliders. However, a way to configure the conversions from model to model in code behind and a way for producing colors are needed.

ColorEditor to be placed in Window which opens on a Buttons's Click. The Button is located inter StandartPalette in RadColorPicker and it is titled More Colors or Edit Colors.
This feature is demonstrated in the new ColoPicker's First Look Demo but it could be implemented out of the box as  well.

when the slider of the LuminancePad is at its minimum or maximum the control visualized wrong color. 

TextBlocks RGB CMYK HSL are cut when the style is extracted from blend. 

As a workaround use an implicit style approach. You can have folder with your XAML files and edit the Style of RadColorEditor in them.
http://docs.telerik.com/devtools/wpf/styling-and-appearance/styling-apperance-implicit-styles-overview