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I am trying to print out a very bright yellow colour but it is coming out mustard
I thought bateman_ap hit it, but it appears you're printing to a standard printer, correct? His advice is still sound, but you do not want to match a m RGB yellow to CMYK values - the two color spaces are too different. What you are looking for is a perfect RGB yellow.
Open in Photoshop, and make sure the Info window is open. place your mouse over the yellow area. You should see R=255, G=255, B=0 in the info window. (and look at what it's saying in CMYK values to demonstrate my point above.)
If you have a clean yellow in the file, the printer's color space is making adjustments to maintain the best average of color across the spectrum and is making your yellow reddish (clean head pending, per thecoalman's comments.) In most cases, this is a GOOD idea - any contamination of yellow tends to make it move toward green.
There may be some software with your printer that can help you adjust for color shifts, but if there are not - you're pretty much stuck with using a combination of the Hue/Saturation controls in your graphics program in combination with the info box to adjust the values of the file to match expected output.