Not building your car with a specific class in mind will put you at a disadvantage... class wise.
The cars are classed from the current tech sheets handed in that day... car color doesn't matter.
If you are just having fun with it... your good to go.
If your want to be in a more competitive class I would suggest making the best use of the allowable mods in a particular class of your choose.
For maximum fun... Get the rule book out along with the wrenches.