When you heal a boss with an attack (spell or rune) the color of the healing Is white, for example SD to Sir Nictros.
If you heal a boss with a UH, IH, or exura gran mas res, the color is that clasic red/orange colour you see when you heal yourself or a friends. For example Leiden with mas res.
Only few bosses can be healed by players, the most commons are Leiden from Cults of Tibia and the white ball in Lord Azaram from Grave Danger in Carlin.