Isn't there a rule somewhere that the final form can't/shouldn't be the same color as the base form?

Not really. We have some where the color is different and some where the color is pretty much the same.

The last I can think of that went like that was Double, though.

Double had it. Den-O liner form was still Red with white and Ryuki Survive changed his undersuit to black but his armor turned red.

I was actually going to do a full list in my previous post before I remembered that the only two pre-Decade Heisei Riders I've actually seen are Blade and Ryuki. ><

Oh yeah and Hyper Kabuto still kept red as his primary color with some more silver added to the armor :)

HYPER CLOCK UP!

Sorry... Just had to do it :P

I hope Mach gets a final form, because Dead Heat doesn't look like a final form. Same with Chaser.

They wont. There are no new toys coming out for them.

Kamen Rider Chaser and Dead Heat are supposed to be the ultimate forms for Gou and Chase.

http://i.imgur.com/xQ4KX77.jpg

Am I late...?

Well The Movie Rider has two forms, a thumb drive like thing can make Drive + Mashin Chaser

AND Type Tridoron!

EDIT : I am indeed late, oh well