I think painting them is a bad idea, in that the paint will wear off
 (think of how much use front panel toggles get). Is it possible to get
 the raw plastic for such pritners in different colours? 
 It is, though most of the colors in which they are available are quite
 garish.  See: 
http://ultimachine.com/catalog/print-materials/pla/pla-3mm
 for a representative sample.  
 
So nothing suitable for DEC handles, I guess.
  Also, how strong are 3D pritned parts comapred to
injection-moulded
 ones?The original ones do break so if the replacements are significantly
 weaker, they're not going to last. 
 They're not as strong.  But there's also no tooling costs, and once
 you've printed up one, you can print up a million for the cost of the
 plastic (probably less than $1 each for a hollow switch that size).  
 
Sure, I understand what 3D printing is. But actually, if you are going to
make a million of them, it'd probably be worth making the mould and
having them injection-moulded (yes, I know tooling costs are very
expensive for such things but for a million...). I doubt there's a market
for anything liek that number.
And yes, 3D printing woudl allow you to make repalcement handles for DEC
frontpanels, but if they break after a few operations (of a few hundred
operations), is  there any real point? They might be OK for a static
dispaly, but I for one use my frontpanels.
-tony