Hi.
The three two stroke bikes I aquired recently (two of them CT's) came with full oil tanks - one with green and two with red oil, but i assume the colour is not a reliable way of identifying oil type. Now I need to top up. Is it OK to just drain the tanks and refill with the oil of my choice? I would normally choose mineral oil for these bikes.
Advice appreciated.  Thanks!

Most injector two stroke oils mix just fine as it is one of the "standards" they must meet...  some "specialty" oils don't play well with others and you don't know what someone used before so I almost always "flush" the system...  basically what you said but you must "bleed" the system anytime you drain the tank or it runs dry.  (See manual if you don't know what this means) 

When changing oils I leave the bleed screw out longer than I normally would to 1) make sure all the air is out of the tank hose, and 2) flush as much old oil from the system as possible. 

Red is a unusual dye color for "garden variety" two stroke oil that should mix.  So another thing I've done more than once is take an ounce or two of the oil in the tank and and mix it in a clear container with the new oil I'm going to use...  mix well...  then let it sit overnight.  Only one time did I have a visual separation in the oils the next morning.  That time I ran the bike on premix after the above procedure with the pump cable pulled to max and the pump outlet hose in a container until I was pretty sure all the old oil was flushed out of the system.  (don't forget to cap the engine side where pump hose connects or you'll have a vacuum leak)

Thanks for comments. Appreciated.

the colour doesn't mean anything in terms of oil performance or base. I chose different colours for 2 stroke and gearbox oil, so I easily con see, where it's coming from, if there is a leak. 

Not too relevant in your case nut back in the mid "70s our AG100B models came with a huge oil tank hung under the big farm carrier & had a flexible filler hose thing.
For some reason we had more than one farmer using a purple almost black twostroke oil that went like porridge over time & blocked the tank outlet.
Was a big oil tank & took ages before it needed topping up.
It had a smaller sight glass guessing 500ml tank lower down which gave you a safety buffer if you saw oil heading past the window it was almost past time to fill up.
Trouble was, oil color stained the glass & no-one probably remembered to look anyway.
So we had seizures.
Oil turned out to be a premix outboard oil they were using. Not sure why or who recommended it--but wasn't us!!.
Best stick to recommended oils for autolube pumps.
Found a couple of pics where you can see the oil tank under the carrier & the lower "Final Warning" sight glass mini tank in the right side pic..
I see the top tank has a glass as well.
photos.app.goo.gl/X8ES7qdSXSJg8Hhk7