Hi ya,
you CAN show them in coloured, but you will be placed WAY down the bottom of the line
Most judges do not class spotties as coloureds. Most appaloosa classes are run either as affiliated classes - i.e. only for members of either BApS or ApHCUK or open - meaning anyone can exhibit their spottie. Sadly, these latter are becoming few and far between.
What 'type' of horse will your youngster be i.e. stocky hunter type, or sports horse type - you could always do what we did and overstamp with Sport Horse GB if the type is right and then show against solids. If your horses conformation is good enough, it will do ok (allowing for spotted bias amongst many judges) but expect to find yourself at the bottom of the lineup unless the conformation is right up there.
What is the breeding of your spottie, how old (sorry, think I saw a pic recently..on here?_).
On another note, did you know there are Appaloosa classes at "The Royal" this year for the first time and that Atira's filly who is by The Mendip Stud's Private B Riker has qualified!!!
If I can help with showing your app, let me know.
Jude.