Hello. This is a quick and easy Guide for those who are bored of seeing the same colours in the Campaign.
I will tell you how you can change them to any other default colour scheme.
For this we will need a few tools:

The SGAExplorer[skins.hiveworldterra.co.uk]
The Texture Tool[skins.hiveworldterra.co.uk]
And optionally any kind of Image Editing Program

Open the SGAExplorer and Click on "File" in the upper left corner.
Open the file DXP2Data.sga in the DXP2 folder of Dark Crusade. There you take the following path:
data-->scenarios-->sp-->races
Here you find the scar files for the different races. Extract the ones you want to change to default.
Now you can go to the same location in your Dark Crusade folder and open the files.

You are searching for the following line: Team Colour = "Default_x"

Now you simply change the number to the corresponding one of your chosen faction taken from this list:

Chaos Marines
default_0 = Alpha Legion (SS default)
default_1 = Word Bearers (DC default)
default_2 = Iron Warriors
default_3 = Night Lords
default_4 = Emperor's Children
default_5 = Black Legion
default_6 = World Eaters
default_7 = Death Guard

Eldar
default_0 = Biel-Tan
default_1 = Saim-Hann
default_2 = Alaitoc
default_3 = Iyanden
default_4 = Menaan
default_5 = Ulthwe (DC and SS default)
default_6 = Ava-Nile
default_7 = Il-Kaithe

Imperial Guard
default_0 = 412th Cadian
default_1 = 180th Cadian
default_2 = 110th Cadian
default_3 = 101st Vendoland
default_4 = 85th Cadian, 252nd Kaurava Conservators (SS default)
default_5 = 51st Coronan Grenadiers
default_6 = 31st Harakoni Warhawks
default_7 = 8th Cadian

Necrons
default_0 = Kronus (DC default), Kaurava III (SS default)
default_1 = Jarrman Primus
default_2 = Lorn V
default_3 = Mefara Secundus
default_4 = Miskal Prime
default_5 = Rahe
default_6 = Sevara
default_7 = Verdus Prime

Orks
default_0 = Blood Axes
default_1 = Bad Moons
default_2 = Goffs
default_3 = Deathskulls
default_4 = Evil Suns (DC and SS default)
default_5 = Big Choppas
default_6 = Gold Toofs
default_7 = Snakebites

Space Marines
default_0 = Blood Ravens (DC and SS default)
default_1 = Ultramarines
default_2 = Space Wolves
default_3 = Imperial Fists
default_4 = Dark Angels
default_5 = Blood Angels
default_6 = Raven Guard
default_7 = White Scars

Tau Empire
default_0 = T'au (DC default)
default_1 = Dal'yth
default_2 = Bork'an
default_3 = D'yanoi
default_4 = Fal'shia (SS default)
default_5 = Sa'cea
default_6 = Tau'n
default_7 = Vior'la

Now you save the changes. Your units should now have the chosen colour in the normal skirmish missions of the campaign.
What will remain unchanged are the Stronghold and Special Missions as well as the commander on the wargear screen and planet map. Changing those is a bit more complicated.

Changing the Strongholds of the four vanilla races (Eldar,SM,Orks,CSM) is as easy as changing the race files. Simply extract the files titeld "stronghold_(race).scar" from the folder above the race files. You'll want all of them. While your at it, you should also extract the files for the four special missions which are: "cl_eres_badlands.scar" "cl_hyperion_peaks.scar" "cl_pavonis.scar" and "cl_vandea_coast.scar".

Now you open those files and search for the line "Misc_PlayerTeamColor(g_PlayerY,"default_X")". Above that will be the information what race it belongs to. Change the X to the same number as you did in the race file. If you changed the colour of any vanilla race you also want to insert the correct number in the corresponing race paragraf above this.

Changing the Colour of the Necrons, Tau and Guard is more complicted and requires use of the Texture Tool. The Process is as follows:

1. With the SGAExplorer you open the file "DXP2Data-SharedTextures-Full"
2. Now you take the path Data-->art-->ebps-->races and then the race you want to change
3. You only need the .wtp files. Easiest way is to extract all files and filter for the wtp's in the folder, then delete the other files.
4. Go back to the DXP2Data.sga file and find the colour schemes of your faction under
data-->art-->team_colours and extract the colour scheme you need
5. You will also need the badges and banners. Those can be found in the folders above.
6. Now its time to open the Texture Tool. Click on "Edit" and choose "Advanced Mode". Then click on "Options" and import the Teamcolour you need under "Teamcolour Settings". After that you click on "Badge" and "Banner" and choose the corresponding ones. To find these you follow the same path as you did in the SGAExplorer. Finish by clicking on "Ok".
7. With the correct Scheme imported you click on "Compile TGA" and choose all the WTP files. Then convert the TGA files with "TGA->DDS". Finally turn those into .rsh files by choosing "Make RSH". Those are the files the game loads for the stronghold missions.

Your Commander will already have the correct colour inside the missions. But if you also want to change them for the wargear screen and the Map you will have to do the same WTP to RSH process as before, so if you did this for any DLC race, then its commander will already show the correct scheme.

The WTP files for the other Commanders and the colour schemes are found in the same place as before but for the badges and banners you will need to open the file "W40kData" in the "W40k" folder.

Changing the Commander on the planetary map is the most complicated process.

1. Open you image edititing program.
2. Open the .dds files leftover from making the .rsh files for the wargear screen
3. Change the size of the image to 256x256
4. Export the image, with DXT1 compression and using the existing mipmaps.
5. Rename it, adding mm somewhere in the file name (this should be done while exporting the image)

Where the mm is differs from file to file. Simply search for the original files with the SGAExplorer and name your new ones accordingly.

Now you need to extract one of the original mm-commander files with the Texture Tool to get the file "background_illum". Put a copy of this file in the same folder as all the others.
Now you click on "Make RSH" and choose your fresh mm-file. The Texture Tool will ask if you want to include additional maps. Select "Yes" and choose the "background_illum" file.

And with that you are done. All models in the game should now be of the new colour you chose.

If you want to change your units to a Custom Colour you made, please consult this Guide.

It is also the place I got most of the information found here from, so special thanks go out to Schweineschnute, Kasrkin84 and R.I.P.