White candles can be substituted for any other color. The only advice I would give is that if you substitute white for another color I'd substitute all candles that you want to attach equal weighting to for white too or you may push more energy through the colored candle by mistake.
Birthday candles can be used but I wouldn't actually recommend them that much myself, they have a relatively short burn time which may not be enough for some rituals, the last thing you want is for the candle to burn out during the spell. Whilst the magic is about the intent not the tool you use you have have to make sure that you're comfortable using it, if you feel as if the birthday candles are fine go with it, if on the other hand you find them small and inappropriate then it's probably better to emit candles entirely as they're only throwing you off.
If appropriate to your situation you can always make your own candles with beeswax and various dyeing compounds. I find that usually works out cheaper than buying individual candles personally but I'm lucky enough to have a relative that keeps bees so I don't know the actual cost of most wax.