Sometimes if I'm having trouble finding the candle color I need (or I need it quickly), I use a white jar candle and adhere colored tissue paper to the outside of the jar with mod podge. White candles are also pretty universal to any working as long as your intent is clear.
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.
I know several people that use birthday candles; it works just fine unless you think you need it to burn for an extended period.
I also make jar candles. The initial supplies aren't cheap, but I've been using the same $20 block of white was for over a year. I use broken crayons to color them (food coloring does NOT work BTW), and add a few drops of essential oil if I want them scented.
If you have a few large candles that have burned down in the middle to make a well, you can also cut off the extra wax on top and melt that to make a new candle.