I think the method used would apply to all of the Britkits Diesels, I don't know if it would apply to other equipment.
I'll put it here, and Hawk can decide. I'll not assume anything and make this just for the Britkits series of diesels for now.
The directory tree is; Britkits/EMD/RailVehicles/Diesel/SW1200/YourModel/Engine/ It would appear that you can work this with the Alco series as well. In the Engines folder there are 4 sets of files that determine the font, color, and series of numbers used, and the number count is embedded in the .GeoPcDx file.
Firstly, set the number series (e.g. 18xx, 32xx, 4xxx, etc.) in the Engine folder by editing the SW1200_numbers.dscv file with any text editor (notepad works fine). there will be a similarly named file in any of the Engines folders, starting with the loco name. e.g. GP9_numbers.dscv, etc. In this file is the numbering list. You must enter 4 digit series for now, but they can be any series you wish. The Sw1200f uses a sequence starting with 1800 and skips on to 1825. You don't need to enter numbers in a series, or even start with 0. Each number is in an .xml set bounded by <CSVItem>...</cCSVItem> you can limit the series size by deleting any of these sets. Be VERY careful not to delete the </cCSVArray> entry at the end of the list! Save this file.
Change to the /Textures folder. Within that folder, you will find a 'SW1200_digits.bin' file. Open it in RW_Tools, or drag it to Serz.exe to convert it to and editable .xml file and open it in a text editor. Within the file, you will find 10 entries like this; <TextureID d:type="cDeltaString">Britkits\EMD\RailVehicles\Diesel\SW1200\Default\Engine\Textures\[00]number_0</TextureID>. Change the \Default\ entry for all 10 instances to \YourModel\ foldername. Save the file. If you don't have RW_Tools, reconvert it to a .bin by dragging and dropping it on Serz.exe.
If you haven't done so already, you'll need to create a folder in the Railworks/Source directory with your name upon it. Add a subdirectory called 'RailVehicles' and a subfolder within that called 'Graphics'.
There are 10 files with the actual graphics, number_0.TgPcDx to number_9.TgPcDx. I use a graphics converter called 'ConvIm.exe' (
http://www.btinternet.com/~mnwright/programs/convim.htm) for graphics conversion. Convert the files to 32 bit .TGA files and save them to your Yourname\RailVehicles\graphics\ folder. Open each of the files in a suitable graphics editor (I use Gimp) and change the colors to suit using the 'bucket fill' utility. The number sets are on a invisible background, and can be changed to a different font if you wish, the size and positioning are critical, and must match closely. In my case I found Arial to be acceptable for my use and didn't need to change them. You may want to add a mask layer to cut down the brightness before saving them. Save all 10 of the files as .tga files. Once completed reconvert them to .ace files using the ToAce utility from Railworks. once the .ace conversion is complete, open the Blueprint Editor within Railworks and navigate to YourName/RailVehicles/Graphics/ Right click on each of the .ace files (they have a paintbrush icon) and select 'Export. Once you've completed all 10 files, close the blueprint editor.
Go back to the main Railworks directory and open the Assets folder. Find the YourName folder (wait, I didn't do that! MAGIC!) open RailVehicles/Graphics/ and you'll find the 10 .TgPcDx files you just converted, and three others with the same name for each one. I usually just copy all 10 files to the desktop. Go back out to the Assets folder, go to Britkits/EMD/RaiVehicles/Diesel/Sw1200/YourName/Textures/. create a folder called 'WhiteNumbers' in the texture folder. drag each of the 'number_x.TgPcDx' files to that folder so you save them. Lasso all the 'number_x.TgPcDx' on your desktop and right click and move them to the Textures folder.
This should give you the colored numbers and fonts you created. Have fun.