Hi, You certainly can by coloured dyes for cement, that can be used as an admixture. I would try source the powered type rather than the liquid as the colour fast is better. I rendered my fireplace about 8 years ago. I also had solid fuel burner installed, standard render was okay and still no signs of adverse effects to heat. I wouldn't put you off trying to find a heat resistive product but I havn't found it a problem as yet.
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