It has been asked many times, and has been on my to-do list for ages, but finally the concept of ‘Any Color’ parts has been added.
The part color called [No Color] has been renamed to [No Color/Any Color], to better match it's new meaning. In official LEGO Set inventories it's used for parts that don't have a specific color (e.g. sticker sheets). Previously, when used in MOCs it was treated as an error. Now, when used in MOCs it indicates that the part can be any color you like as it won't have any impact on the final model - usually because the part can't be seen.
Other Rebrickable features including Buy Parts and the Build engine can now use this information to improve their results, as described below.
No Color/Any Color Parts
To modify a part, simply click it, choose the Edit tab and pick the color at the bottom of the drop down list where the “special” colors are. Note that the [Unknown] color is still treated as an error and should be fixed.
To make it more obvious which parts are being treated specially, they will appear in the default Blue color in inventories.
When searching external BricLink/BrickOwl/ToyPro stores, parts in [No Color/Any Color] will be matched against the cheapest available colors for each store automatically. If the store needs to fulfil the quantity requested by using multiple colors, it will. So for example if you ask for 1000 x[No Color/Any Color] parts you might get back something like the image below. This extends to the Multi-Buy page too.
Clicking the Add to Cart buttons will cause those [No Color/Any Color] parts to be added as whatever real colors the store had in stock. The same thing applies when adding parts to the external site wish lists/wanted lists.
Note that BrickLink and BrickOwl differ slightly in how they show the parts. BrickLink will show the (Not Applicable) color so you don't actually know what you will get until you add it to the store's cart. BrickOwl shows the actual color the store has in stock.
The Build engine has also been updated to handle these parts differently. It will match the [No Color/Any Color] parts against any available parts you have in your buildable collection. This works regardless of the Color Matching option you are using.
This all relies on the MOC designers modifying their existing MOC inventories and new submissions to use this color when relevant. In some cases, designers are already using unusual colors (e.g. Pink) to indicate this so a simple Bulk Edit can update them all in one hit. For an example of one already done, you can checkout MOC-18916 The Empire over Jedha City. If you are looking to Buy/Build a MOC and want to modify the parts yourself, I suggest copying the parts to a Custom List where you have full control over everything. If it’s a Public Custom List, you can even post a link to it in the comments of the MOC to aid other users or even the designer in updating the inventory.